feat(ef): implement the calendarium's bissextile February shift

The Missale Romanum's calendarium footnote for February (LT.txt:5011-5014,
scan-corroborated scan2.txt:3050-3058): in a leap year the sixth kalends of
March (24 February) is doubled rather than a 29th day appended, so St
Matthias moves 24->25 February and St Gabriel of Our Lady of Sorrows
27->28, with 24 February itself carrying no fixed office that year.
data/ef/expected-divergences-lms.sexp entry L3 tracked this as an open gap.

Implemented as the rubric's own general mechanism (every fixed entry from
24 through 28 February shifts one civil day later in a leap year), not as
"move these two named saints": the two readings are indistinguishable on
shipped data (nothing else is fixed in that window) and diverge only for a
future --overlay entry in the same range, which the mechanism reading
covers correctly and a two-saints special case would not. This project has
already been bitten once (RG 16(a)) by a rule implemented against shipped
data's coincidental shape rather than the rubric itself.

Kernel stays rite-agnostic: Rite.t gains a fixed_key field (Date.t -> (int
* int) option), the same seam easter already establishes, read only by
Layer.on_date's FIXED half -- the movable half (Easter_offset/Nth_weekday)
is untouched. Default is the identity mapping, an optional argument on
on_date, so every existing caller and every rite that supplies nothing is
byte-identical to before this field existed. Date.is_leap is exposed in
date.mli (already existed in date.ml) so the rite reads the kernel's own
single-sourced leap-year definition. The EF implementation lives in
Rite_ef.Temporal_ef.bissextile_fixed_key, fully cited, wired into
Rite_ef.context.

Blast radius, measured over the full 1583-9999 domain (Calendar.year run
twice per civil year, real fixed_key vs identity, every day diffed): all
2,041 leap years in the domain show a visible change for each saint;
6,983 individual liturgical days change total, zero unclassified, across
four shapes (Matthias vacates 24 Feb in 1,803 years / occupies 25 Feb in
1,811; Gabriel vacates 27 Feb in 1,699 / occupies 28 Feb in 1,670).
L3's own prior estimate (1,650 years, observed-outright only) is confirmed
close on the same definition (1,677 measured); the broader observed-or-
commemorated count is higher, not lower, showing the narrow estimate
undercounted visible impact rather than overcounting it.

The lectio differential (2005-2050 fixture) gains a new cited entry, C40
in data/ef/expected-divergences.sexp: lectio implements no such shift, so
every leap year in its window now diverges on the two shifted days (21
rows, not the naive 44 -- the comparator never compares commemorations, so
a side where the shifted saint has zero comparator-visible footprint on
both engines produces no row). The LMS Ordo's 2023-2024 fixture -- the
window L3 was originally found in -- now matches on the Creed comparison;
its formulary-override bucket count is corrected 182->181 (24 February
leaves the counted Proper population, becoming Own_slug-sourced). The
missalemeum oracle fixtures (2026-2027, 2038, 2035) cover no leap year and
are unaffected.

L3 is closed: converted from an active allow-list record to a prose
closure paragraph (L1's own precedent), citation preserved verbatim, fix
and measured blast radius recorded. The id-list assertion narrows to [L4]
alone, and the now-dead "2024-02-24" -> "L3" date mapping is removed.

Six golden values pinned across four new test cases: Matthias in a leap
year (both the vacated and occupied sides) and a common-year control;
Gabriel likewise, deliberately choosing a leap year where he is admitted
only as an ordinary commemoration rather than winning outright, a
different shape from Matthias's.

Mutation-tested: forcing fixed_key back to identity reddens exactly five
test cases -- the differential's Layer C count-pin (C40 drops to 0 actual
vs 21 expected), the LMS Ordo's Creed and formulary-bucket checks, and
both new golden pins -- confirming the tests actually exercise the fix.
Reverted; dune test, the exhaustive sweep, and make check are all green.

1 files changed, 70 insertions, 1 deletions

diff --git a/test/test_golden.ml b/test/test_golden.ml
index ba25d8a..d680720 100644
--- a/test/test_golden.ml
+++ b/test/test_golden.ml
@@ -1315,6 +1315,67 @@ let test_rogation_wednesday_yields_to_a_feast_2026 () =
     "2026-05-13 wednesday season=paschaltide week=6 slug=ef-ascension-vigil rank=class-2 colour=white \
      subject=temporal name_la=- comms=[robert-bellarmine:ordinary] in=- out=[]"
 
+(* ---- The bissextile (leap-year) calendarium footnote, bissextile-shift
+   task, 2026-08-22 ----
+
+   The Missale Romanum's own CALENDARIUM table, February, footnote
+   (docs/research/LT.txt:5011-5014, scan-corroborated docs/research/
+   scan2.txt:3050-3058): "In anno bissextili mensis februarius est dierum
+   29, et festum S. Matthiae celebratur die 25 februarii, ac festum S.
+   Gabrielis a Virgine perdolente 28 februarii, et bis dicitur sexto
+   calendas, id est die 24 et die 25" -- in a leap year the sixth kalends
+   of March (24 February) is doubled rather than a 29th day appended, so
+   St Matthias (ordinarily 24 February) is kept on 25 February and St
+   Gabriel of Our Lady of Sorrows (ordinarily 27 February) on 28 February;
+   24 February itself carries no fixed office that year.
+   {!Rite_ef.Temporal_ef.bissextile_fixed_key} implements this as the
+   rubric's own general MECHANISM, so a leap year moves the VACATED date's
+   slug/rank/colour away and the OCCUPIED date's toward it -- both sides
+   pinned for each saint, plus one common-year control apiece proving the
+   identity default is unchanged where the footnote does not apply. *)
+
+(* Matthias, 2044 (leap): 24 February carries no fixed office at all
+   (Class4 temporal only, zero commemorations); Matthias (Class2, an
+   Apostle) wins 25 February outright against that day's own Class4
+   feria. *)
+let test_bissextile_matthias_leap_2044 () =
+  check ~msg:"leap year: 24 February carries no fixed office, Matthias has moved away" 2044 2 24
+    "2044-02-24 wednesday season=septuagesima week=2 slug=ef-septuagesima-2-wednesday rank=class-4 colour=violet \
+     subject=temporal name_la=- comms=[] in=- out=[]";
+  check ~msg:"leap year: Matthias observed outright on 25 February, not 24" 2044 2 25
+    "2044-02-25 thursday season=septuagesima week=2 slug=matthias rank=class-2 colour=red subject=saint name_la=- \
+     comms=[] in=- out=[]"
+
+(* Matthias, 2025 (common): unaffected control -- the identity default
+   applies, Matthias stays at his ordinary 24 February. *)
+let test_bissextile_matthias_common_2025 () =
+  check ~msg:"common year: Matthias unaffected, still 24 February" 2025 2 24
+    "2025-02-24 monday season=septuagesima week=2 slug=matthias rank=class-2 colour=red subject=saint name_la=- \
+     comms=[] in=- out=[]"
+
+(* Gabriel of Our Lady of Sorrows, 2008 (leap): 27 February carries no
+   trace of him at all (not even a commemoration -- fixed_key removes him
+   from the candidate set entirely, RG 111's admission machinery never
+   sees him); on 28 February he is admitted as an ORDINARY commemoration
+   (Class3 ties the day's own privileged Lenten feria, which keeps
+   observed). A different shape from Matthias's outright win above,
+   deliberately pinned: the mechanism moves WHERE the saint is tried, not
+   whether he wins once he gets there. *)
+let test_bissextile_gabriel_leap_2008 () =
+  check ~msg:"leap year: 27 February carries no trace of Gabriel, he has moved away" 2008 2 27
+    "2008-02-27 wednesday season=lent week=3 slug=ef-lent-3-wednesday rank=class-3 colour=violet subject=temporal \
+     name_la=- comms=[] in=- out=[]";
+  check ~msg:"leap year: Gabriel admitted as an ordinary commemoration on 28 February, not 27" 2008 2 28
+    "2008-02-28 thursday season=lent week=3 slug=ef-lent-3-thursday rank=class-3 colour=violet subject=temporal \
+     name_la=- comms=[gabriel-of-our-lady-of-sorrows:ordinary] in=- out=[]"
+
+(* Gabriel of Our Lady of Sorrows, 2025 (common): unaffected control --
+   observed outright at his ordinary 27 February. *)
+let test_bissextile_gabriel_common_2025 () =
+  check ~msg:"common year: Gabriel unaffected, still observed on 27 February" 2025 2 27
+    "2025-02-27 thursday season=septuagesima week=2 slug=gabriel-of-our-lady-of-sorrows rank=class-3 colour=white \
+     subject=saint name_la=- comms=[] in=- out=[]"
+
 (* RG 128, transcribed in docs/research/rules-register.md §3b: violet is used
    for "II/III-class vigils outside Paschaltide". Two of colitur's five vigils
    disagreed with that rule until the ef-oconnell-rubrics branch -- the
@@ -1510,5 +1571,13 @@ let suite =
       Alcotest.test_case "RG87: Rogation Wednesday admitted (2024)" `Quick
         test_rogation_wednesday_admitted_2024;
       Alcotest.test_case "RG87/113: Rogation Wednesday yields to an impeded feast (2026)" `Quick
-        test_rogation_wednesday_yields_to_a_feast_2026
+        test_rogation_wednesday_yields_to_a_feast_2026;
+      Alcotest.test_case "bissextile footnote: Matthias moves 24->25 February (leap, 2044)" `Quick
+        test_bissextile_matthias_leap_2044;
+      Alcotest.test_case "bissextile footnote: Matthias unaffected (common, 2025)" `Quick
+        test_bissextile_matthias_common_2025;
+      Alcotest.test_case "bissextile footnote: Gabriel of Our Lady of Sorrows moves 27->28 February (leap, 2008)"
+        `Quick test_bissextile_gabriel_leap_2008;
+      Alcotest.test_case "bissextile footnote: Gabriel of Our Lady of Sorrows unaffected (common, 2025)" `Quick
+        test_bissextile_gabriel_common_2025
     ] )