test(differential): split the Holy Family/Baptism collision out of C1 (C15)

Before this task's Holy Family fix, colitur's generic Sunday
fallback lost outright to the fixed Commemoration of the Baptism of
the Lord on 13 January whenever it fell on a Sunday, producing the
SAME slug lectio's own tridentine-calendar.ini shows there --
squarely inside C1's own 6-13 January blanket (only Season differed).
Now that Holy Family (RG 17(b)) correctly outranks the fixed Baptism
and excludes it as a commemoration (RG 91 entry 14, RG 112(a)),
colitur's own slug on those seven dates (2008, 2013, 2019, 2030,
2036, 2041, 2047) changes to ef-time-after-epiphany-sunday-1 -- a
genuine identity divergence against lectio, which has no Holy Family
at all. C1's own predicate does not absorb it (jan_6_13_slug has no
entry for that slug), so it surfaced as unexplained, exactly the
check the task brief asked for: split it out into its own cited
entry rather than widening C1's predicate to swallow it silently.

C1's own expected_rows moves 368 -> 361 (back to its pre-rebootstrap
value, for a third and different reason than either of the first two
moves recorded in its own note); the new C15 covers the 7 rows,
gated on the literal date list AND colitur's own slug, the same
identity guard C1/C6/C8/C14 already apply.

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