fix(of-lectionary): stop serving drifting readings for O-Antiphon and Christmas-season dates

The bootstrap excluded 19 lectio bases as "date-keyed duplicates", but
17 of them were not duplicates at all: the 8 O-Antiphon days
(17-24 December) and 8 further Christmas-season dates (29-31 December,
2-5 and 7 January) each carry unique per-date content found nowhere else
among the emitted entries, verified against lectio's own ini directly.
17 December is Gen 49:2,8-10 / Matthew 1:1-17, but colitur was falling
through to that year's unrelated weekday-keyed "advent-3-mon" content
instead.

OLM n. 69.3 explains why: those ferias are fixed by civil date, unlike
every other Advent/Christmastide/Paschaltide feria, which is merely
non-alternating within a weekday slot. Temporal_of's own ferial slugs
for these dates are weekday-keyed, so looking one of those up served
whichever OTHER date happened to share that year's weekday alignment --
a citation that silently drifted year to year on ~16 real civil dates
(6 January stays excluded: Temporal_of fixes Epiphany there
unconditionally, so that date can never reach the ferial path at all;
two more, easter-6-thu and advent-4-sat, stay excluded for their own,
different structural reasons).

Fixed with a date-keyed lookup route (Lectionary_of.date_keyed_slug),
tried before the weekday-keyed one in readings' own step 3.
Temporal_of's slugs are unchanged -- only which lectionary key resolves
a day's citations. tools/bootstrap_lectionary_of.ml now maps the 17
bases to synthetic date-keyed slugs instead of excluding them, and its
own reachability sweep calls date_keyed_slug directly so the two can
never drift apart. Re-emitted data/of/lectionary.sexp (754 -> 770
entries) and corrected its provenance header, which previously
described these dates as excluded duplicates.

Also states, for the first time, a Minor finding from the same review:
190 of 222 shipped sanctoral slugs have no dedicated lectionary entry
and correctly fall through to the ferial per OLM norms -- expected
behaviour, not a gap, but never said in the coverage report before.

New tests pin 17 December (Gen 49:2,8-10 / Matthew 1:1-17) and 2 January
(1 John 2:22-28 / John 1:19-28) in two years with different weekday
alignments, proving neither reading drifts, plus direct boundary
coverage of date_keyed_slug itself (both windows, the 6 January
exclusion, and the Sunday guard).

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