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, 18 insertions, 0 deletions

diff --git a/test/test_oracle.ml b/test/test_oracle.ml
index 7ee3848..f1d1a89 100644
--- a/test/test_oracle.ml
+++ b/test/test_oracle.ml
@@ -1878,6 +1878,21 @@ let classify_2038 (c : colitur_row) (o : oracle_row) diffs =
   else if contains title "Mass of the B. V. M." then Some "BVM-SATURDAY-MASS (M26 shape 1)"
   else if first_is "Col 1:12-20" then Some "CHRIST-THE-KING-WEEK (M26 shape 2b)"
   else if diffs = [ Colour_f ] && List.mem c.c_observed_slug m21_2038_slugs then Some "COLOUR (M21)"
+  (* RG 33's third omission trigger (register 6.31, implemented
+     2026-08-21). The Sacred Heart takes 24 June 2038, so the Nativity of St
+     John the Baptist is transferred under RG 96 and the vigil of 23 June is
+     "penitus omissa". missalemeum keeps it -- as does lectio, whose own C39
+     covers this same date on the differential side, which is exactly what
+     one expects of two points on one lineage. Gated on the ORACLE's own
+     title rather than on the date, so a different divergence arriving on 23
+     June could not be silently absorbed here. Cited to the register rather
+     than to an allow-list id because data/ef/expected-divergences-missalemeum.sexp
+     is scoped to the 2026-2027 window, which contains no instance of this
+     clause -- the same convention PERPETUA-COMMON and MISSAL-TWO-PART-CITATION
+     already follow. The tracked, full account is C39's own note in
+     data/ef/expected-divergences.sexp. *)
+  else if contains title "Vigil of the Nativity of St. John the Baptist" then
+    Some "RG33-THIRD-TRIGGER (colitur; register 6.31, see C39)"
   else if first_is "Judith" then Some "JUDITH (M28, register 6.6)"
   else if c.c_observed_slug = "sts-felicitas-perpetua" then Some "PERPETUA-COMMON (register 6.8)"
   else if c.c_observed_slug = "ef-nativity" then Some "CHRISTMAS-MULTI-MASS (M27)"
@@ -2108,6 +2123,9 @@ let test_2038_class_counts () =
         ("COMM-ONLY-TIE-BREAK (RG-under-determined convention -- register 6.16)", 1);
         ("JOSEPH-ANNUNCIATION-TRANSFER-ORDER (C14's own collision)", 2);
         ("JUDITH (M28, register 6.6)", 2);
+        (* RG 33's third trigger: 23 June only -- the Sacred Heart takes 24
+           June 2038 and St John the Baptist transfers. See M31. *)
+        ("RG33-THIRD-TRIGGER (colitur; register 6.31, see C39)", 1);
         ("MISSAL-TWO-PART-CITATION (register 6.9)", 1);
         ("COMM-IDENTITY-UNRESOLVED (temporal-origin, no English name -- M15's limit)", 2);
         ("PERPETUA-COMMON (register 6.8)", 1);