test(differential): two Layer C predicates that counted rows, not identities

Both are the "cardinality where identity was required" flavour, and both
were demonstrated passing a real defect by the fixture-refresh review
rather than argued.

C6 accepted `subset diffs [ Slug_f; Rank ]` while its own note said the
shape had narrowed to slug-only after lectio's Christmas-octave rank was
corrected. That did not merely over-permit: every one of the 138 rows
already carries a Slug_f diff, so an added Rank diff changed neither the
diff-set membership nor the count, and the entry was blind. Dropping the
RG 91 entry 17 elevation in temporal_ef (Class2 -> Class4 on octave days
5-7) left both differential tests green with C6 still reporting 138.
Narrowed to `diffs = [ Slug_f ]`; the same mutation now reddens both.

C14 gated on lectio's date alone, with no colitur-slug guard, though fix
round 1's finding 1 had established that a predicate must pin which
celebration its citation is about -- C1, C6 and C8 all carry that guard.
With compare_deferred switched to RG 98 origin order the observed
celebration flips to the Annunciation on all three dates, and C14 still
matched exactly 3 rows with its count pin green; the suite reddened only
because the sibling dates fell outside every predicate. Guard added.

No behaviour change: 277 tests green, and the allow-list counts are
unmoved (C6 138, C14 3).

1 files changed, 26 insertions, 4 deletions

diff --git a/test/test_differential.ml b/test/test_differential.ml
index 62c7a48..89e6873 100644
--- a/test/test_differential.ml
+++ b/test/test_differential.ml
@@ -485,15 +485,37 @@ let layer_c_reason (l : row) (c : row) diffs =
   else if
     m = 12
     && (d = 29 || d = 30 || d = 31)
-    && subset diffs [ Slug_f; Rank ]
-    && (not (List.mem Slug_f diffs) || List.mem c.slug nativity_octave_day_slugs)
+    (* NARROWED, fixture-refresh review: this was [subset diffs [ Slug_f; Rank ]].
+       Since lectio's own Christmas-octave rank was corrected (3b32c00) all 138
+       rows diff on [Slug_f] ALONE, and the note below says so -- but leaving
+       [Rank] in the accepted set did not merely over-permit, it made the entry
+       BLIND: every row already carries a Slug_f diff, so an added Rank diff
+       changed neither the diff-set membership nor the count. Demonstrated by
+       the reviewer: dropping the RG 91 entry 17 elevation in temporal_ef
+       (Class2 -> Class4 on octave days 5-7) left BOTH differential tests green
+       with C6 still reporting 138. The slug guard below already pins identity;
+       [Rank] only removed the ability to notice a rank regression. *)
+    && diffs = [ Slug_f ]
+    && List.mem c.slug nativity_octave_day_slugs
   then Some "C6"
   else if
     (String.equal c.slug "ef-rogation-monday" || String.equal c.slug "ef-rogation-tuesday")
     && subset diffs [ Season; Slug_f; Colour_f ]
   then Some "C8"
-  else if List.mem l.date joseph_annunciation_collision_dates_2005_2050 && subset diffs [ Slug_f; Rank ] then
-    Some "C14"
+  else if
+    List.mem l.date joseph_annunciation_collision_dates_2005_2050
+    && subset diffs [ Slug_f; Rank ]
+    (* Colitur-slug guard, added by the fixture-refresh review. Fix round 1's
+       own finding 1 established that a Layer C predicate must pin WHICH
+       celebration the citation is about, not merely the date -- C1, C6 and C8
+       all carry that guard; C14 was written after that ruling and omitted it.
+       Demonstrated live: with [Calendar.compare_deferred] switched to RG 98
+       origin order, the observed celebration on all three dates flips to the
+       Annunciation, and C14 still matched exactly 3 rows with its count pin
+       green. The suite only reddened because the SIBLING dates fell outside
+       every predicate -- geometry, not this pin. *)
+    && String.equal c.slug "joseph-spouse-of-the-bl-virgin-mary"
+  then Some "C14"
   else None
 
 (* ---------------------------------------------------------------------- *)