test(differential): fix round 3 -- two more Ember Saturdays, doc fixes

Two Ember Saturdays shipped an apostle's Mass verbatim from lectio's
own ini, the exact shape already found and fixed for Passion Tuesday
(C26): ef-advent-ember-sat carried St Thomas the Apostle's Mass (Eph
2:19-22 / John 20:24-29, 21 December) instead of the real "Sabbato
Quatuor Temporum Adventus" (2 Thess 2:1-8 / Luke 3:1-6); ef-september-
ember-sat carried St Matthew's Mass (Ezek 1:10-14 / Matt 9:9-13, 21
September) instead of the real "Sabbato Quatuor Temporum septembris"
(Heb 9:2-12 / Luke 13:6-17). Both verified against both primary-source
scans independently. Both readings are explicitly labelled "Lectio
Epistolae" in the Missal on both days, so neither needed the "last
lesson before the Gospel" convention Holy Wednesday/Good Friday relied
on -- a textual fact, not an editorial choice.

Fixed at the source in tools/bootstrap_lectionary.ml: colitur_keys
excludes both wrong ini sections; a new ember_saturday_corrections list
hand-authors the real citations. New allow-list entries C27/C28 (40
rows each, not the naive 46 -- traced to the identical 6 civil years
where 21 September and 21 December, always exactly 91 days apart, land
on a Saturday and the fixed saint's feast wins outright). Both values
pinned in test_lectionary.ml and mutation-tested.

Also: C25's per-weekday breakdown was mislabelled (right multiset,
rotated weekday labels) in both test_differential.ml and expected-
divergences.sexp -- corrected to Wednesday 6/Thursday 19/Friday
19/Saturday 13, and the claim that this split came directly from the
comparator's own output is narrowed to what's actually true (only the
total, 57, does). C26's scan2 citation range was extended to include
the Gospel (previously stopped after the Epistle).

lib/kernel/liturgical_day.mli's "citations always empty until Plan 4"
comment is now stale on this branch -- left untouched (Task 11's own
documentation pass), noted in the task report so it isn't lost.

dune test: 358 tests green; COLITUR_EXHAUSTIVE_SWEEP=1: 359 green.

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