test(differential): gate C1/C6 on slug family, pin the fixture's SHA-256

Fix round 1 on Task 15's review, four findings closed.

C1 and C6 previously gated on calendar date alone -- unlike every other
Layer C entry, which also checks the actual colitur slug that won. A
future sanctoral change producing an unrelated celebration with the same
diff shape (e.g. a promoted 29-31 December saint landing on C6's own
[Slug_f; Rank]) would have been silently absorbed under a citation that
has nothing to do with the real cause. Both entries now require, whenever
Slug_f is in the diff set, that colitur's own slug be a member of an
explicit closed list (jan_6_13_slug, nativity_octave_day_slugs) -- the
same style advent_feria_slug already used for C4. Constructed the exact
substitution via a temporary (unshipped) overlay Replace directive
promoting thomas-becket to Class1 Feast White; confirmed the tightened
predicate refuses to absorb it (46 unexplained rows, one per year); reverted.

C10 gained the diff-field-subset restriction every other entry already
had (it previously fired on the two pinned dates regardless of which
fields differed).

Fixed a stale doc comment ("SIX" columns; field has five constructors).

The fixture's provenance note now records its SHA-256, and the suite
asserts it as its own first test case (test_fixture_checksum), so a
hand-edited or partially re-copied fixture fails loudly instead of
silently becoming an unlabelled snapshot. Computed via the system
sha256sum binary through Sys.command plus a redirected-output temp file
-- no new library dependency, not even unix. Constructed a one-byte
fixture edit and confirmed the checksum test fails independently of the
row-comparison test; reverted.

237/237 tests green, clean-build verified.

0 files changed, 0 insertions, 0 deletions