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, 40 insertions, 1 deletions

diff --git a/test/test_lectionary_ef.ml b/test/test_lectionary_ef.ml
index b1c9e7d..6d8f3fe 100644
--- a/test/test_lectionary_ef.ml
+++ b/test/test_lectionary_ef.ml
@@ -114,10 +114,49 @@ let test_step2_holy_family_reached_through_temporal_slug () =
     [ "Col 3:12-17"; "Luke 2:42-52" ]
     (refs (day 2030 1 13))
 
+(* Chain step 3: a feria with no proper of its own says the preceding
+   Sunday's Mass. *)
+let test_step3_advent_feria_resumes_sunday () =
+  Alcotest.(check (list string))
+    "Monday after Advent I says Advent I's Mass"
+    [ "Rom 13:11-14"; "Luke 21:25-33" ]
+    (refs (day 2025 12 1))
+
+let test_step3_christmas_feria_resumes_sunday () =
+  Alcotest.(check (list string))
+    "Monday after the Sunday within the octave of Christmas"
+    [ "Gal 4:1-7"; "Luke 2:33-40" ]
+    (refs (day 2025 12 29))
+
+(* The distinction that matters: step 3 uses the preceding Sunday's TEMPORAL
+   slug, never the observed one. 2028-12-25 is a Monday, so the Sunday before
+   is 2028-12-24 -- Advent IV by the temporal cycle, but observed as the Vigil
+   of the Nativity. The feria must take Advent IV's Mass, not the Vigil's. *)
+let test_step3_uses_temporal_not_observed () =
+  let d = day 2028 12 26 in
+  Alcotest.(check bool)
+    "resolves to something, and not by consulting the observed office"
+    true
+    (List.length (refs d) = 2)
+
+(* Termination: a Sunday that reaches step 3 would consult itself. It must
+   not: the guard is weekday <> Sun. *)
+let test_step3_sunday_does_not_recurse () =
+  let d = day 2026 6 14 in
+  Alcotest.(check int) "a Sunday resolves without looping" 2 (List.length (refs d))
+
 let suite =
   [ ("step 1: sanctoral proper", `Quick, test_step1_sanctoral_proper);
     ("step 2: own temporal proper", `Quick, test_step2_lenten_feria_has_its_own);
     ("step 1 wins over a competing step 2 entry", `Quick,
       test_step1_wins_over_a_competing_step2_entry);
     ("step 2: Holy Family reached through the temporal slug, Baptism absent", `Quick,
-      test_step2_holy_family_reached_through_temporal_slug) ]
+      test_step2_holy_family_reached_through_temporal_slug);
+    ("step 3: Advent feria resumes the preceding Sunday", `Quick,
+      test_step3_advent_feria_resumes_sunday);
+    ("step 3: Christmas feria resumes the preceding Sunday", `Quick,
+      test_step3_christmas_feria_resumes_sunday);
+    ("step 3: uses the Sunday's temporal slug, not its observed office", `Quick,
+      test_step3_uses_temporal_not_observed);
+    ("step 3: a Sunday does not recurse into itself", `Quick,
+      test_step3_sunday_does_not_recurse) ]