ef(lectionary): a feria says the preceding Sunday's Mass

Chain step 3. Guarded on weekday <> Sun: a Sunday reaching this branch
would look up its own slug via days_since_sunday Sun = 0 and loop --
every other chain step consults data, this one consults a strictly
earlier date, so that guard is the whole chain's termination argument.
Reaches the Sunday by Date.add_days plus a fresh temporal_at call, never
by string surgery on the day's own slug -- the slug shapes are genuinely
inconsistent across seasons (ef-advent-sunday-1 vs ef-advent-1-monday,
week number on opposite sides of the season name).

Uses the preceding Sunday's TEMPORAL slug, never its observed one: the
rubric is the preceding Sunday's Mass even in a year a feast displaced
that Sunday from being observed (pinned: 2028-12-26, the Monday after a
Vigil-displaced Advent IV Sunday, still takes Advent IV's Mass).

Measured over the full 1583-9999 domain (temporal cycle only, no
sanctoral contest): of 412 distinct temporal slugs, 305 carry no
lectionary entry of their own; of the 304 that are feria (non-Sunday)
slugs, step 3 alone resolves 297 of them via their preceding Sunday.
The 7 that remain, plus the 1 uncovered Sunday slug itself
(ef-holy-name-sunday), all trace to the same two missing lectionary
entries (Holy Name Sunday and 30 December), not to eight independent
gaps or a step-3 defect -- traced date-by-date, not merely counted.
Day-level effect, 2005-2050 (full Precedence+Calendar pipeline,
matching this project's existing differential window): 16807 days,
16531 resolved (98.36%), 276 still empty; step 3 alone accounts for
4503 of the resolved days, more than either step 1 or step 2.

Warrant is the same class as step 2's, not a confirmed Missal
citation: lectio hard-codes this shape as literal duplicated data on
the four Advent ferias (Advent I's own readings copied onto the
following Monday-Thursday) and leaves the rest of that same shape
simply absent; step 3 turns the duplication into a rule.

dune test --force: 344 tests, all green (was 340).

1 files changed, 55 insertions, 2 deletions

diff --git a/lib/rites/rite_ef/lectionary_ef.ml b/lib/rites/rite_ef/lectionary_ef.ml
index ec1aa0a..02c3c64 100644
--- a/lib/rites/rite_ef/lectionary_ef.ml
+++ b/lib/rites/rite_ef/lectionary_ef.ml
@@ -2,6 +2,10 @@ open Colitur_kernel
 
 (* Step 1: the observed celebration's own proper.
    Step 2: the day's own temporal slug.
+   Step 3: a weekday whose own slug has no entry says the preceding Sunday's
+   Mass -- see the implementation comment on that branch in [readings] for
+   the termination argument and why it is the Sunday's TEMPORAL, not
+   observed, identity.
 
    Nothing here encodes "Lent has daily propers": the presence of an entry in
    [lectionary] is the sole discriminator -- this function does not branch on
@@ -17,10 +21,59 @@ open Colitur_kernel
    ferial-Mass rubrics when coding") -- that confirmation has not been done;
    do not read this comment as citing RG/the Missal for the SELECTION rule
    itself, only [Lectionary.find]'s presence-or-absence as the mechanism. *)
-let readings ~lectionary ~observed ~temporal ~date:_ ~temporal_at:_ =
+(* Days from a given weekday back to the preceding Sunday. Sunday itself
+   yields 0, which is why step 3 must guard on it -- see [readings] below. *)
+let days_since_sunday : Date.weekday -> int = function
+  | Date.Sun -> 0
+  | Date.Mon -> 1
+  | Date.Tue -> 2
+  | Date.Wed -> 3
+  | Date.Thu -> 4
+  | Date.Fri -> 5
+  | Date.Sat -> 6
+
+let readings ~lectionary ~observed ~temporal ~date ~temporal_at =
   match observed.Celebration.citations with
   | _ :: _ as cs -> cs
   | [] -> (
       match Lectionary.find lectionary temporal.Temporal.office.Celebration.slug with
       | Some cs -> cs
-      | None -> [])
+      | None -> (
+          (* Step 3: a feria with no proper of its own says the preceding
+             Sunday's Mass. WARRANT is the same as step 2's -- lectio's own
+             observed behaviour, not a confirmed Missal citation: this is the
+             rule lectio hard-codes as data on the four Advent ferias
+             (Advent II's readings copied verbatim onto the following
+             Monday-Saturday) and leaves absent on the other slugs this step
+             now also reaches; docs/research/rules-register.md already
+             records the ferial-Mass selection rule itself as unconfirmed
+             against the primary source.
+
+             Termination: guarded on weekday. A Sunday reaching this branch
+             would compute [days_since_sunday Sun = 0] and look up ITSELF,
+             looping forever -- every other step in the chain consults data
+             (a lectionary lookup), this is the only one that consults
+             another date, so this guard is the whole chain's termination
+             argument, not merely a special case.
+
+             The preceding Sunday's TEMPORAL slug, never its observed one:
+             the rubric is the preceding Sunday's Mass even in a year when a
+             feast displaced that Sunday from being observed (see
+             test_step3_uses_temporal_not_observed). [temporal_at] gives the
+             temporal identity of any date, so the Sunday is reached by date
+             arithmetic and a fresh call to the temporal cycle -- never by
+             string surgery on [own_slug]: the slug shapes are genuinely
+             inconsistent across seasons (e.g. [ef-advent-sunday-1] versus
+             [ef-advent-1-monday], the week number on opposite sides of the
+             season name), so deriving one from the other textually would be
+             a latent bug the moment a season's naming convention differs. *)
+          let offset = days_since_sunday temporal.Temporal.weekday in
+          if offset = 0 then []
+          else
+            let sunday = Date.add_days date (-offset) in
+            let sunday_temporal = temporal_at sunday in
+            match
+              Lectionary.find lectionary sunday_temporal.Temporal.office.Celebration.slug
+            with
+            | Some cs -> cs
+            | None -> []))