feat(ef): RG 111(a), the sung-Mass commemoration cap

Item 1 of Phase 3 (celebrant-rubrics-phase1): the "at Low Mass"
commemoration-placement rule the design spec recorded as unread. It is
not a new rule -- it is the sung/low axis of RG 111, which colitur
already implements.

RG 111(a) (LT.txt, "Ratio admittendi commemorationes"): a liturgical
day of the first class, AND any non-conventual sung Mass regardless of
the day's own class, admits at most one commemoration, and only if it
is privileged. (b)/(c)/(d), the same rubric's remaining clauses, give
the wider caps colitur's admit already computes -- which is exactly
the LOW MASS answer.

Exposed as Precedence.sung_mass_commemorations, a pure derivation over
the existing Low-Mass admitted list (filter to Privileged, keep the
first), not a new stored field on Liturgical_day.t: the input list is
already validated and privilege-tagged, so a second field would only
create a second place for the same fact to drift out of sync with the
first, with no new information gained. Liturgical_day.t.commemorations
is now documented as the Low Mass set explicitly, removing the
ambiguity its .mli previously left unstated.

Tested against synthetic Low-Mass sets (none/one/two privileged,
already-first, empty) and two real calendar days resolved through the
normal Cal.day pipeline: 2026-08-14 (Vigil of the Assumption, an
ordinary-only commemoration, dropped at Sung Mass) and 2026-04-25 (the
Major Litanies, RG 80/109(f), privileged, kept at both Masses).

1 files changed, 37 insertions, 1 deletions

diff --git a/test/test_rubrics_ef.ml b/test/test_rubrics_ef.ml
index 6679702..d533670 100644
--- a/test/test_rubrics_ef.ml
+++ b/test/test_rubrics_ef.ml
@@ -746,6 +746,38 @@ let test_exhaustive_gloria_domain_sweep () =
     Alcotest.(check bool) "the full domain reached a real number of Requiem days" true (black > 1_000)
   end
 
+(* ---- ITEM 1: RG 111(a), the sung-Mass commemoration cap
+   ({!Colitur_kernel.Precedence.sung_mass_commemorations}) -- two real
+   calendar days, one of each shape, resolved through the identical
+   [Cal.day] pipeline every other test in this file uses. See
+   precedence.mli's own citation for the rubric and the reasoning for why
+   this is a derivation over [LD.commemorations] (the Low-Mass admitted
+   set), not a second stored field. ---- *)
+
+module Prec = Colitur_kernel.Precedence
+
+let sung_slugs y m d =
+  List.map
+    (fun (c, _) -> Slug.to_string c.Cel.slug)
+    (Prec.sung_mass_commemorations (day_on y m d).LD.commemorations)
+
+let test_rg111a_ordinary_only_dropped () =
+  (* 2026-08-14: Vigil of the Assumption, commemorating [eusebius-confessor]
+     ORDINARILY (no privileged category applies) -- confirmed via `colitur
+     day 2026`. At Low Mass this is admitted (RG 111(c)); at a
+     non-conventual Sung Mass RG 111(a) admits it not at all. *)
+  Alcotest.(check (list string)) "RG 111(a): an ordinary-only Low-Mass set is empty at Sung Mass" []
+    (sung_slugs 2026 8 14)
+
+let test_rg111a_privileged_kept () =
+  (* 2026-04-25: St Mark, commemorating the Major Litanies -- RG 80/109(f),
+     a PRIVILEGED commemoration (data/ef/adjustments.sexp's own [Add
+     major-litanies]) -- confirmed via `colitur day 2026`. Kept at BOTH Low
+     Mass (RG 111(c), one privileged) and Sung Mass (RG 111(a), the same
+     single privileged commemoration). *)
+  Alcotest.(check (list string)) "RG 111(a): the one privileged commemoration survives at Sung Mass"
+    [ "major-litanies" ] (sung_slugs 2026 4 25)
+
 let suite =
   ( "Rubrics_ef",
     [ Alcotest.test_case "475(a): ordinary Sunday" `Quick test_475a_ordinary_sunday;
@@ -819,4 +851,8 @@ let suite =
       Alcotest.test_case "domain sanity: every Requiem day has [gloria]=false (sample)" `Quick
         test_domain_requiem_implies_no_gloria_sample;
       Alcotest.test_case "domain sweep 1583..9999: violet/Requiem invariants, committed not sampled"
-        `Slow test_exhaustive_gloria_domain_sweep ] )
+        `Slow test_exhaustive_gloria_domain_sweep;
+      Alcotest.test_case "RG 111(a): an ordinary-only Low-Mass set is empty at Sung Mass" `Quick
+        test_rg111a_ordinary_only_dropped;
+      Alcotest.test_case "RG 111(a): a privileged commemoration survives at Sung Mass" `Quick
+        test_rg111a_privileged_kept ] )