test(differential): allow-list C12 for the 15-entry rank correction

Re-bootstrapping sanctoral.sexp (previous commit) makes 7 of the 15
newly-corrected III-class feasts win an occurrence they previously lost as
Commemoration_only: RG 91 entry 24 (a universal III-class feast) outranks
entry 28 (a IV-class feria), and the occurrence code that resolves this
already existed and needed no change. lectio's own committed differential
fixture (test/fixtures/lectio-ef-2005-2050.txt) is a frozen snapshot
predating lectio's own equivalent generator fix, so it still shows the
class-4 feria observed with the same saint only as a trailing
commemoration -- a new, expected divergence from that fixture, 14 rows in
the 2005-2050 window.

expected_rows (14) is independently re-derived by grepping the regenerated
sanctoral.sexp's own post-regeneration civil-day output against the
committed lectio fixture directly (two flat-file extractions cross-checked
against each other), not transcribed from the comparator's own failing-test
output.

C12 reuses an id last used, and removed, by the RG16(a) task's own reverted
Purification retag -- noted at both the old and new entries' own comments
so the id's history is visible to a future reader.

1 files changed, 22 insertions, 4 deletions

diff --git a/test/test_differential.ml b/test/test_differential.ml
index e12cd9a..cf5b95f 100644
--- a/test/test_differential.ml
+++ b/test/test_differential.ml
@@ -413,6 +413,23 @@ let jan_6_13_slug slug =
 let nativity_octave_day_slugs =
   [ "ef-nativity-octave-day-5"; "ef-nativity-octave-day-6"; "ef-nativity-octave-day-7" ]
 
+(* C12 (ef-rebootstrap, 2026-08-12): the exact 15 slugs data/ef/sanctoral.sexp
+   re-bootstrapped from `rank = commemoration` to a real class-3 feast (the
+   source's own SHA-256 moved 6a25e634... -> 1b303ef2...) -- a literal list,
+   not a date-range or slug-prefix pattern, per the SAME discipline the
+   comment above this one already states for C1/C6 (fix round 1, finding 1):
+   a slug outside this exact set must fail through to [None], not be
+   silently absorbed by a citation about a different set of saints. Only 7
+   of the 15 are ever actually reachable here (the other 8 never collide
+   with a plain class-4 feria anywhere in 2005-2050), but the guard is
+   written against the full set colitur's own data now carries, not the
+   narrower subset this one 46-year window happens to exercise. *)
+let fixed_iii_class_reclassified_slugs =
+  [ "benedict"; "cyril-of-jerusalem"; "forty-holy-martyrs-of-sebaste"; "frances-rome";
+    "francis-of-paola"; "gabriel-the-archangel"; "gregory-the-great"; "isidore-of-seville";
+    "john-damascene"; "john-of-capistrano"; "john-of-god"; "patrick"; "sts-felicitas-perpetua";
+    "thomas-aquinas"; "vincent-ferrer" ]
+
 (* [layer_c_reason l c diffs] returns the [data/ef/expected-divergences.sexp]
    [id] this row-pair's remaining (post Layer A/B) diff set belongs to, or
    [None] if nothing here explains it (a genuine, uncovered failure). *)
@@ -456,10 +473,11 @@ let layer_c_reason (l : row) (c : row) diffs =
      the user has ruled: follow the oracle (register §6.0 has the full
      account). The Purification is `Lord` again, colitur observes it
      outright on those six 2-February Sundays, matching lectio's own
-     `class = lord` exactly -- no divergence to allow-list. Left
-     unimplemented here deliberately, not merely absent by oversight: this
-     comment is the record that C12 existed and was removed, should a
-     future reader wonder why the numbering skips it. *)
+     `class = lord` exactly -- no divergence to allow-list. That C12 was
+     removed, not merely unused -- superseded by the DIFFERENT C12 below,
+     ef-rebootstrap (2026-08-12), reusing the freed id. *)
+  else if List.mem c.slug fixed_iii_class_reclassified_slugs && subset diffs [ Slug_f; Rank; Colour_f ]
+  then Some "C12"
   else None
 
 (* ---------------------------------------------------------------------- *)