Merge branch 'ef-plan3': Plan 3, the EF resolution engine

Adds the rite-parameterised Precedence resolver, Liturgical_day, Rite,
and Calendar (year as the primitive, because transfers need whole-year
knowledge), the full EF precedence ruleset (RG 91's 28-entry table,
occurrence RG 92-95, commemorations RG 108-111, transfers RG 96-98),
322 bootstrapped sanctoral entries, colitur day <year>, and validation
layers 3-5.

Layer 3 diffs 16801 days against lectio; layer 4 diffs 730 days against
missalemeum; layer 5 pins ~30 dates on the known-tricky years. Both
comparison layers carry cited allow-lists that name the governing RG
paragraph and which engine is right.

The oracle layer earned its place immediately: Holy Thursday was violet
in colitur and lectio alike, because colitur's data was bootstrapped
from lectio and both carried the same error. Only an independent source
could see it. RG 128(b) and RG 122 name it white.

1 files changed, 585 insertions, 0 deletions

diff --git a/test/test_differential.ml b/test/test_differential.ml
new file mode 100644
index 0000000..a9bdb91
--- /dev/null
+++ b/test/test_differential.ml
@@ -0,0 +1,585 @@
+(* Task 15: differential harness vs lectio (sibling project, Go), EF only,
+   2005-2050 -- validation layer 3 of the design spec's five (colitur CLAUDE.md
+   "Validation" section; layers 1-2, Types and Property, are already built and
+   green). lectio's own EF stream is committed as a fixture
+   (test/fixtures/lectio-ef-2005-2050.txt, provenance -- including the
+   SHA-256 this file's own [test_fixture_checksum] asserts, so a hand-edit or
+   partial re-copy of the fixture fails loudly rather than silently becoming
+   an unlabelled snapshot -- in the sibling .provenance file next to it);
+   colitur's side is recomputed fresh from this library on every run, through
+   the SAME pipeline `colitur day` uses (Colitur_kernel.Calendar over the
+   real data/ef/sanctoral.sexp + adjustments.sexp), not the compiled binary.
+
+   *** THREE STATED LIMITS THIS COMPARATOR DOES NOT PRETEND TO EXCEED ***
+
+   1. Commemorations are NOT compared. lectio's trailing "+other-slug" tokens
+      are the LOSING sanctoral candidates for the day (its own dumper's doc
+      comment says so), not an RG 111 admitted set -- lectio has no RG 111
+      admission logic at all. Comparing that column would compare colitur's
+      real admitted commemorations against lectio's rejects, which proves
+      nothing. Only the seven leading columns (date weekday season week slug
+      rank colour) are read from either stream.
+
+   2. lectio's own EF oracle test (~/git/projects/lectio,
+      internal/calendar/oracle_ef_test.go) asserts ONLY season, strictly,
+      against missalemeum, and ONLY for 2025-2026. Rank and colour are logged
+      there, never asserted. So lectio's advertised "0-error vs references
+      2005-2050" covers season-exactness for the EF, not full-row-exactness.
+      A rank or colour difference below is therefore NOT presumptive evidence
+      that colitur is wrong -- every one is adjudicated against the Missal/RG
+      register directly (data/ef/expected-divergences.sexp), never against
+      lectio's say-so. (Layer 4, the oracle differential against missalemeum
+      itself, is what actually validates rank and colour; that is a later
+      task.)
+
+   3. The WEEK column is not compared, at all, on either side. Two reasons,
+      both confirmed by reading lectio's own source
+      (cmd/lectio-ef-dump/main.go's doc comment): first, lectio prints week
+      "0" (rendered "-") "where no season week applies, e.g. named I class
+      feasts and per annum green-season ferias" -- an internal DISPLAY
+      convention of lectio's, not a liturgical fact, so there is nothing to
+      cross-check it against. Second, even where both sides print a real
+      number, the two engines anchor Time-after-Epiphany week numbering
+      differently (colitur: the first Sunday on/after Epiphany; lectio: a
+      fixed offset from 6 January -- register §3c item 5) and the offset
+      between them is NOT constant (it depends on which weekday 6 January
+      falls on, and re-synchronises mid-season), so no clean formula-based
+      check is possible without reimplementing lectio's own algorithm here.
+      Slug identity, rank and colour -- where the real liturgical substance
+      lives -- remain fully and separately compared; only the bare integer
+      is out of scope. See the report for the concrete rows this weakens.
+
+   *** THE THREE-LAYER DESIGN (controller ruling, Task 15 dispatch) ***
+
+   Of 16801 day-pairs (2005-2050), 11206 already match on the seven columns.
+   Of the 5595 that don't, exactly 25 distinct (field-diff) signatures cover
+   all of them (task-15-class-summary.txt). They resolve into three strictly
+   separate layers:
+
+   - Layer A (this file's [norm_season]/[norm_slug]): vocabulary. A
+     declarative, explicit, closed table of naming synonyms that carry no
+     liturgical substance -- lectio's "easter"/"christmas" ARE colitur's
+     "paschaltide"/"christmastide"; a handful of slugs are two different
+     engines' names for the identical office (Christmas Vigil, Holy Name
+     Sunday, the Pentecost-octave Ember days, the two Passiontide weeks).
+     Every entry is a literal string pair, never a pattern -- widening this
+     to a wildcard is exactly how a real bug would get hidden, so it is not
+     done even where it would shorten the table.
+
+   - Layer B ([strip_epiphany_index]): numbering. The ONE case in the whole
+     5595 where a slug's embedded index genuinely cannot be reconciled by a
+     literal table (Time-after-Epiphany's non-constant offset, limit 3
+     above) -- both sides' embedded week digit is stripped to a common
+     family+weekday form before comparing, while rank and colour (which
+     carry no week-index information for an ordinary green-season
+     feria/Sunday) remain fully compared, so a genuine identity bug in this
+     family would still be caught by everything except the digit itself.
+
+   - Layer C (data/ef/expected-divergences.sexp, matched by
+     [layer_c_reason] below): the CITED allow-list. Eleven genuine liturgical
+     disagreements (C11 added by Task 16's oracle work, below), each citing
+     its RG paragraph and stating which engine is right (always colitur,
+     verified against the Missal/register, never against lectio's own
+     behaviour -- "lectio does it differently" is not itself a justification
+     anywhere in this file). This is the ONLY layer that may cover a
+     difference in rank, colour, or which celebration is observed; A and B
+     never do (enforced structurally below: A/B only ever
+     touch the season/slug fields, and Layer C's predicates each require an
+     exact, narrow field-diff SET, not "anything goes").
+
+   13 January (register §6's long-open "Baptism of the Lord" item) is
+   EMPIRICALLY CONFIRMED FIXED, not allow-listed: colitur's slug/rank/colour
+   for 13 January already equal lectio's exactly, in all 46 years (the
+   sanctoral wiring landed in Task 11's "13 Jan now resolves to the Baptism
+   of the Lord" review note). The only residual difference there is season
+   (covered by Layer C's C1, the Jan 6-13 boundary) -- see the report. *)
+
+module Cal = Colitur_kernel.Calendar
+module Layer = Colitur_kernel.Layer
+module Overlay = Colitur_kernel.Overlay
+module LD = Colitur_kernel.Liturgical_day
+module Slug = Colitur_kernel.Slug
+module Date = Colitur_kernel.Date
+module Cel = Colitur_kernel.Celebration
+module Colour = Colitur_kernel.Colour
+module V = Rite_ef.Vocab_ef
+
+(* Same relative paths test_rite_ef.ml/test_sanctoral_ef.ml use: dune test
+   runs from _build/default/test/. *)
+let sanctoral_path = "../data/ef/sanctoral.sexp"
+let adjustments_path = "../data/ef/adjustments.sexp"
+let fixture_path = "fixtures/lectio-ef-2005-2050.txt"
+let allow_list_path = "../data/ef/expected-divergences.sexp"
+
+(* The fixture's own provenance note (test/fixtures/lectio-ef-2005-2050.provenance)
+   records this same digest, so it is discoverable by a reader who never runs
+   the suite. Recorded here too, and ASSERTED (fix round 1, finding 4): a
+   provenance note is enough to REGENERATE the fixture but says nothing about
+   whether the committed bytes still match the commit they claim to come
+   from -- an unnoticed hand-edit or partial re-copy would silently turn the
+   whole oracle into an unlabelled snapshot of whatever someone last ran.
+   Regenerate this constant (and the provenance note's copy) together,
+   deliberately, after re-running the exact command the provenance note
+   names -- never by copying the actual value back in to make a mismatch
+   pass, which would defeat the point of pinning it at all. *)
+let fixture_sha256 = "2ca3eeeda4e7a0406c4d004c1b2003fc0df671aca9af18a1b506543a721c8bac"
+
+(* Same technique tools/bootstrap_sanctoral.ml already uses for this exact
+   purpose (that file's own comment: shelling out to the system's
+   [sha256sum], not an OCaml crypto library -- Task 15's deps are frozen).
+   Unlike that tool, this avoids even the (already-permitted, per that
+   file's own comment, "already in the switch") [unix] library: [Sys.command]
+   plus a redirected-to-file capture needs nothing beyond the Stdlib every
+   dune executable already links. Depends on [sha256sum] being on PATH,
+   which every environment this suite has actually run in (Debian, per
+   CLAUDE.md) provides via coreutils; a machine without it fails this check
+   with a command-not-found exit code rather than silently skipping it. *)
+let sha256_of_file path =
+  let tmp = Filename.temp_file "colitur_differential_sha256" ".txt" in
+  Fun.protect
+    ~finally:(fun () -> try Sys.remove tmp with Sys_error _ -> ())
+    (fun () ->
+      let cmd = Printf.sprintf "sha256sum %s > %s" (Filename.quote path) (Filename.quote tmp) in
+      let rc = Sys.command cmd in
+      if rc <> 0 then Alcotest.failf "sha256sum exited %d for %s (is it on PATH?)" rc path;
+      let ic = open_in tmp in
+      let line =
+        try input_line ic
+        with End_of_file ->
+          close_in ic;
+          Alcotest.failf "sha256sum produced no output for %s" path
+      in
+      close_in ic;
+      match String.index_opt line ' ' with
+      | Some i -> String.sub line 0 i
+      | None -> Alcotest.failf "unexpected sha256sum output for %s: %S" path line)
+
+let real_layer () =
+  let layer =
+    match Layer.load V.rank_of_sexp sanctoral_path with
+    | Ok l -> l
+    | Error e -> Alcotest.failf "%s: failed to load: %s" sanctoral_path e
+  in
+  let overlay =
+    match Overlay.load V.rank_of_sexp adjustments_path with
+    | Ok o -> o
+    | Error e -> Alcotest.failf "%s: failed to load: %s" adjustments_path e
+  in
+  let layer, diagnostics = Overlay.apply layer overlay in
+  Alcotest.(check (list string)) "the committed overlay applies cleanly, no diagnostics" []
+    (List.map Overlay.diagnostic_to_string diagnostics);
+  layer
+
+(* --- The seven-column row both streams share (commemorations excluded, --- *)
+(* limit 1 above). *)
+type row = {
+  date : string;
+  weekday : string;
+  season : string;
+  week : string;
+  slug : string;
+  rank : string;
+  colour : string;
+}
+
+let read_lines path =
+  let ic = open_in path in
+  let rec loop acc =
+    match input_line ic with
+    | line -> loop (line :: acc)
+    | exception End_of_file ->
+        close_in ic;
+        List.rev acc
+  in
+  loop []
+
+let row_of_line line =
+  match String.split_on_char ' ' line with
+  | date :: weekday :: season :: week :: slug :: rank :: colour :: _others ->
+      { date; weekday; season; week; slug; rank; colour }
+  | _ -> Alcotest.failf "malformed fixture line (fewer than 7 fields): %S" line
+
+let lectio_rows () = List.map row_of_line (read_lines fixture_path)
+
+(* Recomputes colitur's day-by-day output straight from the library --
+   exactly [Colitur_kernel.Calendar.year] + [Rite_ef.context] over the real
+   committed data, the same pipeline bin/main.ml's `colitur day` runs (that
+   file's own [day_report]/[day_line] comments explain the two-liturgical-
+   year-per-civil-year indexing this mirrors). Duplicated here rather than
+   shared with bin/main.ml (an executable, not a library) -- the same choice
+   test_rite_ef.ml already made for [real_layer] above. *)
+let colitur_rows_2005_2050 () =
+  let layer = real_layer () in
+  let by_rata : (int, (V.season, V.rank) LD.t) Hashtbl.t = Hashtbl.create 20000 in
+  for y = 2004 to 2050 do
+    let days = Cal.year Rite_ef.context layer y in
+    Array.iter (fun (d : (V.season, V.rank) LD.t) -> Hashtbl.replace by_rata (Date.to_rata d.LD.date) d) days
+  done;
+  let mk y m d = match Date.make ~year:y ~month:m ~day:d with Ok t -> t | Error e -> failwith e in
+  let rows = ref [] in
+  for y = 2005 to 2050 do
+    let d = ref (mk y 1 1) in
+    let stop = mk y 12 31 in
+    while Date.compare !d stop <= 0 do
+      (match Hashtbl.find_opt by_rata (Date.to_rata !d) with
+      | Some day ->
+          let t = day.LD.temporal in
+          let cel = day.LD.observed in
+          let week =
+            match t.Colitur_kernel.Temporal.week with Some n -> string_of_int n | None -> "-"
+          in
+          rows :=
+            { date = Date.to_iso8601 day.LD.date;
+              weekday = Date.weekday_to_string t.Colitur_kernel.Temporal.weekday;
+              season = V.season_to_string t.Colitur_kernel.Temporal.season;
+              week;
+              slug = Slug.to_string cel.Cel.slug;
+              rank = V.rank_to_string cel.Cel.rank;
+              colour = Colour.to_string cel.Cel.colour
+            }
+            :: !rows
+      | None -> Alcotest.failf "internal error: no resolved day for %s" (Date.to_iso8601 !d));
+      d := Date.add_days !d 1
+    done
+  done;
+  List.rev !rows
+
+(* ---------------------------------------------------------------------- *)
+(* Layer A: vocabulary. Explicit, closed tables; no wildcards, no pattern  *)
+(* matching against arbitrary content -- every case below is a literal    *)
+(* string compared against a literal string.                             *)
+(* ---------------------------------------------------------------------- *)
+
+(* lectio's season word -> colitur's season word. The ONLY two seasons that
+   are spelled differently; every other season word is shared verbatim
+   (both call Advent "advent", Lent "lent", etc.). *)
+let norm_season = function
+  | "easter" -> "paschaltide"
+  | "christmas" -> "christmastide"
+  | s -> s
+
+let weekdays = [ "monday"; "tuesday"; "wednesday"; "thursday"; "friday"; "saturday" ]
+
+(* lectio's slug -> the colitur slug for the SAME office, where the two
+   projects simply chose different names. [month]/[lectio_rank] disambiguate
+   the two cases where lectio reuses one slug for what colitur names two
+   different ways (see each branch's own comment). *)
+let rec norm_slug ~month ~lectio_rank slug =
+  if String.equal slug "vigil-of-christmas" then "ef-nativity-vigil"
+    (* register §6: lectio names its vigils "vigil-of-X" (prefix); colitur's
+       own convention is "X-vigil" (suffix). Same celebration -- confirmed a
+       genuine duplicate in Task 11, where colitur's overlay suppresses
+       lectio's key entirely from the sanctoral layer. *)
+  else if
+    (* "ef-christmas-sunday-0"/"ef-christmas-0-<weekday>" mean TWO different
+       things in lectio depending on which window they fall in: the Sunday
+       within Holy Name week (2-5 January, colitur's own named feast) or an
+       ordinary day within the Nativity Octave (26-31 December, where colitur
+       uses its own ef-nativity-octave-day-N naming instead, Layer C's C6 --
+       that window keeps a genuine RANK difference too, so it must not be
+       absorbed here). Gating on [month = 1] keeps this table from ever
+       touching the December occurrence. *)
+    month = 1
+  then
+    if String.equal slug "ef-christmas-sunday-0" then "ef-holy-name-sunday"
+    else
+      match List.find_opt (fun wd -> String.equal slug ("ef-christmas-0-" ^ wd)) weekdays with
+      | Some wd -> "ef-christmas-1-" ^ wd
+      | None -> norm_slug_rest ~lectio_rank slug
+  else norm_slug_rest ~lectio_rank slug
+
+and norm_slug_rest ~lectio_rank slug =
+  (* Passiontide: lectio never distinguishes Passion week (colitur's week 1,
+     class-3 ferias) from Holy week (colitur's week 2, class-1 ferias) in the
+     slug -- both print "ef-passiontide-0-<weekday>". Rank, independently and
+     strictly compared elsewhere, is what actually tells the two weeks apart
+     (RG 91 entries 22 vs 2/7), so using it here to pick the expected colitur
+     digit does not launder away a genuine identity bug: a colitur bug that
+     mixed up the two weeks would, on the evidence available in this stream,
+     also very likely show up as a rank mismatch of its own. *)
+  match List.find_opt (fun wd -> String.equal slug ("ef-passiontide-0-" ^ wd)) weekdays with
+  | Some wd -> (
+      match lectio_rank with
+      | "class-3" -> "ef-passiontide-1-" ^ wd
+      | "class-1" -> "ef-passiontide-2-" ^ wd
+      | _ -> slug)
+  | None -> (
+      match slug with
+      | "ef-easter-8-wednesday" -> "ef-pentecost-ember-wed"
+      | "ef-easter-8-friday" -> "ef-pentecost-ember-fri"
+      | "ef-easter-8-saturday" -> "ef-pentecost-ember-sat"
+      | s -> s)
+
+(* ---------------------------------------------------------------------- *)
+(* Layer B: numbering (register §3c item 5). The Time-after-Epiphany       *)
+(* week-index embedded in a slug is stripped to a common form on BOTH     *)
+(* sides before comparing -- see limit 3 in this file's header comment    *)
+(* for why a table (Layer A's tool) cannot do this instead.               *)
+(* ---------------------------------------------------------------------- *)
+
+let starts_with ~prefix s =
+  let lp = String.length prefix in
+  String.length s >= lp && String.equal (String.sub s 0 lp) prefix
+
+let is_digit_string s = s <> "" && String.for_all (fun c -> c >= '0' && c <= '9') s
+
+let strip_epiphany_index slug =
+  let prefix = "ef-time-after-epiphany-" in
+  if not (starts_with ~prefix slug) then slug
+  else
+    let rest = String.sub slug (String.length prefix) (String.length slug - String.length prefix) in
+    match String.index_opt rest '-' with
+    | None -> slug
+    | Some i ->
+        let left = String.sub rest 0 i in
+        let right = String.sub rest (i + 1) (String.length rest - i - 1) in
+        if is_digit_string left && List.mem right weekdays then prefix ^ right
+        else if String.equal left "sunday" && is_digit_string right then prefix ^ "sunday"
+        else slug
+
+(* ---------------------------------------------------------------------- *)
+(* Field-diff computation: applies Layers A and B, then reports exactly   *)
+(* which of the FIVE substantive columns still differ (week is never      *)
+(* inspected at all -- limit 3). weekday is included defensively: dates   *)
+(* are checked 1:1 aligned before this runs, so it should never fire, and *)
+(* if it ever does that is real signal, not noise to normalise away.      *)
+(* ---------------------------------------------------------------------- *)
+
+type field = Weekday | Season | Slug_f | Rank | Colour_f
+
+let field_name = function
+  | Weekday -> "weekday"
+  | Season -> "season"
+  | Slug_f -> "slug"
+  | Rank -> "rank"
+  | Colour_f -> "colour"
+
+let month_of_date date = int_of_string (String.sub date 5 2)
+
+let diff_fields (l : row) (c : row) =
+  let m = month_of_date l.date in
+  let l_season = norm_season l.season in
+  let l_slug = strip_epiphany_index (norm_slug ~month:m ~lectio_rank:l.rank l.slug) in
+  let c_slug = strip_epiphany_index c.slug in
+  List.filter_map
+    (fun x -> x)
+    [ (if String.equal l.weekday c.weekday then None else Some Weekday);
+      (if String.equal l_season c.season then None else Some Season);
+      (if String.equal l_slug c_slug then None else Some Slug_f);
+      (if String.equal l.rank c.rank then None else Some Rank);
+      (if String.equal l.colour c.colour then None else Some Colour_f)
+    ]
+
+(* ---------------------------------------------------------------------- *)
+(* Layer C: the cited allow-list (data/ef/expected-divergences.sexp).     *)
+(* Each predicate below names the [id] it matches; the sexp file carries  *)
+(* that id's citation, verdict and expected row count. A predicate fires  *)
+(* only on an EXACT, narrow field-diff set -- never "any difference at    *)
+(* all" -- so it cannot silently absorb a difference outside what its own *)
+(* citation actually explains.                                            *)
+(* ---------------------------------------------------------------------- *)
+
+let subset xs ys = List.for_all (fun x -> List.mem x ys) xs
+let day_of_date date = int_of_string (String.sub date 8 2)
+
+let advent_feria_slug slug =
+  List.exists
+    (fun wk -> List.exists (fun wd -> String.equal slug (Printf.sprintf "ef-advent-%d-%s" wk wd)) weekdays)
+    [ 3; 4 ]
+
+let sunday_iclass_slugs =
+  [ "ef-advent-sunday-2"; "ef-advent-sunday-4"; "ef-lent-sunday-1"; "ef-lent-sunday-2"; "ef-lent-sunday-3" ]
+
+let rose_sunday_slugs = [ "ef-advent-sunday-3"; "ef-lent-sunday-4" ]
+
+(* Fix round 1, finding 1: C1 and C6 (below) originally gated on calendar
+   date alone, with no slug/slug-family check -- unlike every other entry
+   here. The reviewer constructed the failure this leaves open: if a future
+   sanctoral regeneration made some OTHER 29-31 December celebration win the
+   day (colliding coincidentally with C6's own [Slug_f; Rank] diff shape),
+   it would be silently absorbed under "RG 91 entry 17, Nativity Octave" --
+   a citation that has nothing to do with the real cause. [subset diffs
+   [...]] alone was never enough; the SLUG that actually won must also be
+   the one each citation is about. Both lists below are exact literals (the
+   Nativity Octave's three colitur-only day slugs; the closed set of
+   offices that can legitimately observe C1's Jan 6-13 window), not
+   patterns -- a slug outside them fails through to [None] instead of being
+   absorbed. *)
+let jan_6_13_slug slug =
+  String.equal slug "ef-epiphany"
+  || String.equal slug "commemoration-of-the-baptism-of-the-lord"
+  || List.exists (fun wd -> String.equal slug ("ef-christmas-2-" ^ wd)) weekdays
+
+let nativity_octave_day_slugs =
+  [ "ef-nativity-octave-day-5"; "ef-nativity-octave-day-6"; "ef-nativity-octave-day-7" ]
+
+(* [layer_c_reason l c diffs] returns the [data/ef/expected-divergences.sexp]
+   [id] this row-pair's remaining (post Layer A/B) diff set belongs to, or
+   [None] if nothing here explains it (a genuine, uncovered failure). *)
+let layer_c_reason (l : row) (c : row) diffs =
+  let m = month_of_date l.date and d = day_of_date l.date in
+  if diffs = [] then None
+  else if
+    m = 1 && d >= 6 && d <= 13
+    && subset diffs [ Season; Colour_f; Slug_f ]
+    && (not (List.mem Slug_f diffs) || jan_6_13_slug c.slug)
+  then Some "C1"
+  else if List.mem c.slug sunday_iclass_slugs && diffs = [ Rank ] then Some "C2"
+  else if List.mem c.slug rose_sunday_slugs && subset diffs [ Rank; Colour_f ] then Some "C3"
+  else if advent_feria_slug c.slug && diffs = [ Rank ] then Some "C4"
+  else if starts_with ~prefix:"ef-lent-ember-" c.slug && subset diffs [ Slug_f; Rank ] then Some "C5"
+  else if
+    m = 12
+    && (d = 29 || d = 30 || d = 31)
+    && subset diffs [ Slug_f; Rank ]
+    && (not (List.mem Slug_f diffs) || List.mem c.slug nativity_octave_day_slugs)
+  then Some "C6"
+  else if (String.equal c.slug "matthew" || String.equal c.slug "thomas") && subset diffs [ Slug_f; Colour_f ]
+  then Some "C7"
+  else if
+    (String.equal c.slug "ef-rogation-monday" || String.equal c.slug "ef-rogation-tuesday")
+    && subset diffs [ Season; Slug_f; Colour_f ]
+  then Some "C8"
+  else if
+    (String.equal l.slug "joseph-spouse-of-the-bl-virgin-mary"
+    || String.equal c.slug "joseph-spouse-of-the-bl-virgin-mary")
+    && subset diffs [ Season; Slug_f; Rank; Colour_f ]
+  then Some "C9"
+  else if
+    (String.equal l.date "2011-07-02" || String.equal l.date "2011-07-04")
+    && subset diffs [ Slug_f; Rank; Colour_f ]
+  then Some "C10"
+  else if String.equal c.slug "ef-passiontide-2-thursday" && diffs = [ Colour_f ] then Some "C11"
+  else None
+
+(* ---------------------------------------------------------------------- *)
+(* data/ef/expected-divergences.sexp loading -- a plain sequence of        *)
+(* top-level records (not one wrapping list: see the file's own header). *)
+(* ---------------------------------------------------------------------- *)
+
+open Sexplib0.Sexp_conv
+
+type allow_entry = { id : string; citation : string; verdict : string; note : string; expected_rows : int }
+[@@deriving sexp]
+
+let load_allow_list () =
+  let sexps =
+    try Sexplib.Sexp.load_sexps allow_list_path
+    with e -> Alcotest.failf "%s: failed to load: %s" allow_list_path (Printexc.to_string e)
+  in
+  List.map allow_entry_of_sexp sexps
+
+(* ---------------------------------------------------------------------- *)
+(* The comparison itself, run once and shared by every test case below   *)
+(* (Alcotest test cases are independent processes-in-a-list, not free to  *)
+(* share mutable state across a suite, so this recomputes per call --     *)
+(* acceptable: colitur's own 1583-9999 property sweep runs orders of      *)
+(* magnitude more days in tens of seconds, and this is 16801 civil days,  *)
+(* once per test case, twice total). *)
+(* ---------------------------------------------------------------------- *)
+
+type outcome = Matched | Explained of string | Unexplained of field list
+
+let compare_streams () =
+  let lectio = lectio_rows () in
+  let colitur = colitur_rows_2005_2050 () in
+  (lectio, colitur)
+
+let classify lectio colitur =
+  List.map2
+    (fun (l : row) (c : row) ->
+      if not (String.equal l.date c.date) then
+        Alcotest.failf "streams misaligned: lectio %s vs colitur %s" l.date c.date;
+      let diffs = diff_fields l c in
+      if diffs = [] then (l, c, Matched)
+      else
+        match layer_c_reason l c diffs with
+        | Some id -> (l, c, Explained id)
+        | None -> (l, c, Unexplained diffs))
+    lectio colitur
+
+let describe_unexplained (l : row) (c : row) diffs =
+  Printf.sprintf "%s: %s differ -- lectio=(%s %s %s %s %s) colitur=(%s %s %s %s %s)" l.date
+    (String.concat "," (List.map field_name diffs))
+    l.weekday l.season l.slug l.rank l.colour c.weekday c.season c.slug c.rank c.colour
+
+(* Fix round 1, finding 4: the fixture's own byte content must still match
+   the SHA-256 its provenance note claims (lectio commit 2386a45, recorded
+   both here and in test/fixtures/lectio-ef-2005-2050.provenance). Checked
+   before anything else reads the fixture -- a drifted fixture makes every
+   other assertion in this suite a statement about an unlabelled snapshot,
+   not about the pinned lectio commit it claims to be. *)
+let test_fixture_checksum () =
+  Alcotest.(check string) "fixture SHA-256 matches its provenance note" fixture_sha256
+    (sha256_of_file fixture_path)
+
+(* Dates align 1:1 in the same order on both streams (both are one line per
+   civil day, 2005-01-01..2050-12-31 -- see the fixture's own provenance
+   note and [colitur_rows_2005_2050]'s construction). A silent misalignment
+   would make every subsequent comparison meaningless -- checked first, on
+   its own, rather than trusted. *)
+let test_dates_align () =
+  let lectio, colitur = compare_streams () in
+  Alcotest.(check int) "both streams have 16801 rows (46*365 + 11 leap days)" 16801 (List.length lectio);
+  Alcotest.(check int) "colitur recomputed the same number of rows" (List.length lectio) (List.length colitur);
+  let mismatched =
+    List.filter_map
+      (fun (l, c) -> if String.equal l.date c.date then None else Some (l.date, c.date))
+      (List.combine lectio colitur)
+  in
+  Alcotest.(check (list (pair string string))) "no misaligned dates" [] mismatched
+
+(* The core assertion: every one of the 5595 raw differences is either
+   normalised away (Layers A/B) or named in the cited allow-list (Layer C).
+   Nothing else is permitted to pass silently. *)
+let test_no_unexplained_differences () =
+  let lectio, colitur = compare_streams () in
+  let classified = classify lectio colitur in
+  let unexplained =
+    List.filter_map
+      (fun (l, c, outcome) ->
+        match outcome with Unexplained diffs -> Some (describe_unexplained l c diffs) | _ -> None)
+      classified
+  in
+  Alcotest.(check (list string)) "no differences outside Layers A/B/C" [] unexplained
+
+(* Teeth, not just green: EVERY Layer C entry's actual row count over this
+   fixture must equal what data/ef/expected-divergences.sexp declares, in
+   BOTH directions -- an id used by [layer_c_reason] that is missing from
+   the sexp file, an id declared but never matched, or a count that has
+   drifted either up or down, all fail loudly. A silent drift here is
+   exactly the "allow-list absorbs a new bug" failure mode this task was
+   warned about. *)
+let test_layer_c_counts_match_citations () =
+  let lectio, colitur = compare_streams () in
+  let classified = classify lectio colitur in
+  let actual_counts = Hashtbl.create 16 in
+  List.iter
+    (fun (_, _, outcome) ->
+      match outcome with
+      | Explained id ->
+          Hashtbl.replace actual_counts id (1 + Option.value ~default:0 (Hashtbl.find_opt actual_counts id))
+      | _ -> ())
+    classified;
+  let declared = load_allow_list () in
+  let expected =
+    List.sort compare (List.map (fun e -> (e.id, e.expected_rows)) declared)
+  in
+  let actual =
+    List.sort compare
+      (Hashtbl.fold (fun id n acc -> (id, n) :: acc) actual_counts [])
+  in
+  Alcotest.(check (list (pair string int)))
+    "every allow-list id's actual row count matches its citation's expected_rows, and no id is unused or \
+     undeclared"
+    expected actual
+
+let suite =
+  ( "differential (lectio, EF, 2005-2050)",
+    [ Alcotest.test_case "fixture SHA-256 matches its provenance note" `Quick test_fixture_checksum;
+      Alcotest.test_case "streams are 16801 rows each, dates aligned 1:1" `Quick test_dates_align;
+      Alcotest.test_case "every difference is normalised (A/B) or cited (C) -- none unexplained" `Quick
+        test_no_unexplained_differences;
+      Alcotest.test_case "Layer C counts match data/ef/expected-divergences.sexp exactly" `Quick
+        test_layer_c_counts_match_citations
+    ] )