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).

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