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, 6 insertions, 4 deletions

diff --git a/lib/rites/rite_ef/lectionary_ef.mli b/lib/rites/rite_ef/lectionary_ef.mli
index 65fbfe5..159fd57 100644
--- a/lib/rites/rite_ef/lectionary_ef.mli
+++ b/lib/rites/rite_ef/lectionary_ef.mli
@@ -20,10 +20,12 @@ open Colitur_kernel
     masking the gap in every test run). See the task report for the
     reproduction.
 
-    Steps 1 and 2 only (Task 4): the observed celebration's own proper, else
-    the day's own temporal slug in the lectionary. A day matching neither
-    gets [] for now -- the ferial fallback to the preceding Sunday (Task 5)
-    and the Commons (Task 6) are not built here. *)
+    Steps 1-3 (Tasks 4-5): the observed celebration's own proper, else the
+    day's own temporal slug in the lectionary, else -- for a weekday whose
+    own slug has no entry -- the preceding Sunday's temporal slug (never its
+    observed one; a Sunday guards against consulting itself and looping,
+    see [readings]'s own implementation comment). A day matching none of the
+    three gets [] for now -- the Commons (Task 6) are not built here. *)
 val readings :
   lectionary:Lectionary.t ->
   observed:Vocab_ef.rank Celebration.t ->