feat(ef): implement RG 33's third omission trigger

RG 33 omits a II/III-class vigil in three cases: it falls on a Sunday, it
falls on a I-class feast, "vel si festum cui praemittitur in alium diem
transferri aut ad commemorationem reduci contingat". Only the first two
were built; the third was recorded in precedence_ef.ml as unimplemented on
the grounds that no witness existed in the shipped data. That reasoning was
wrong, and the rule fires on 1 744 days across 1583-9999.

Both halves of the clause reduce to one observable question -- is the feast
the OBSERVED office on the following day (RG 34 puts it there) -- so the
kernel asks it once per candidate, after place_transfers has settled the
year. No fixed point is needed: a vigil is a candidate only on its own day,
never on its feast's, so suppressing it cannot change what the next day
observes. Precedence.rules gains vigil_feast, which returns the slug of the
feast a vigil precedes; the kernel cannot infer that itself, because only
two of the five vigil/feast pairs share a slug stem.

Blast radius, measured pre-change binary vs HEAD over the whole domain and
classified: 1 744 days, three shapes, zero unexplained. 1 199 are the feast
reduced to a commemoration (10 August on a Sunday, St Lawrence); 478 and 67
are the feast transferred under RG 96 after the Sacred Heart or Corpus
Christi takes its day. The Assumption's and the Ascension's vigils never
qualify -- their I-class feasts always keep their own day.

Independently witnessed, which is unusual here. The published Ordo -- the
only witness outside the Divinum Officium -> missalemeum -> lectio lineage
-- omits St Lawrence's vigil on 2025-08-09, agreeing with colitur against
both engines. That date had been read earlier as an Ordo gap; the Ordo was
right, and correcting the misreading is what surfaced this clause. On
2027-08-09 the feast does keep its day and the Ordo omits a vigil colitur
correctly keeps, which is a genuine Ordo gap.

Allow-lists: C39 (lectio, 10 rows) and a 2038 oracle class citing the
register, the 2026-2027 window having no instance. The golden pin asserting
St Lawrence's vigil is violet moved 2025 -> 2027; its own comment had
reasoned about the vigil's weekday and missed that RG 33 also looks at the
feast's. Two new pins cover both shapes of the clause.

The vigil/feast table is built with Slug.of_string_exn: mutation testing
showed that of_string plus Result.to_option turns a typo into None, which
this hook's contract reads as "not a vigil", switching the rule off in
silence. Two tests assert the table against the shipped data in both
directions.

1 files changed, 23 insertions, 0 deletions

diff --git a/test/test_differential.ml b/test/test_differential.ml
index 8423888..8c1dd8d 100644
--- a/test/test_differential.ml
+++ b/test/test_differential.ml
@@ -1089,6 +1089,13 @@ let rg69_one_hop_slugs = [ "ef-christmas-1-saturday" ]
 (* [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). *)
+(* The three vigils RG 33's third trigger can actually reach on the shipped
+   universal calendar -- see C39 in [layer_c_reason]. Named here rather than
+   written inline so a future slug rename fails in one visible place. *)
+let rg33_vigil_slugs =
+  [ "vigil-of-st-lawrence"; "vigil-of-the-nativity-of-st-john-the-baptist";
+    "vigil-of-sts-peter-paul" ]
+
 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
@@ -1135,6 +1142,22 @@ let layer_c_reason (l : row) (c : row) diffs =
      colour-only Saturday still falls to C17 and cannot be absorbed here. *)
   else if is_bvm_saturday_row c diffs then Some "C17"
   else if diffs = [ Colour_f ] && List.mem c.slug audit_colour_corrected_slugs then Some "C18"
+  (* C39 -- RG 33's THIRD omission trigger, implemented 2026-08-21. lectio
+     omits a II/III-class vigil that itself falls on a Sunday (the rule's
+     SECOND trigger) but not one whose FEAST fails to keep its own day, so on
+     these days lectio still shows the vigil and colitur no longer does.
+
+     Gated on LECTIO's own slug being one of the three vigils this clause can
+     reach, and on colitur's NOT being a vigil -- identity on both sides, not
+     merely a diff-set shape, the same discipline C6 and C14 were given. The
+     three are exhaustive here: the Assumption's and the Ascension's vigils
+     can never qualify, because their I-class feasts always keep their own
+     day (measured over the whole 1583-9999 domain, 0 occurrences). *)
+  else if
+    List.mem l.slug rg33_vigil_slugs
+    && (not (List.mem c.slug rg33_vigil_slugs))
+    && subset diffs [ Slug_f; Rank; Colour_f; First_f; Gospel_f ]
+  then Some "C39"
   (* C25 -- CLOSED, BVM Saturday Mass (2026-08-17), predicate removed: 0 rows.
      Its days are unoccupied IV-class Saturdays, so they carry RG 78's office
      and now RG 309(a)'s Mass for it, which answers before step 3's walkback