calendar: pull December's sanctoral calendar (pre-1955 pull, 12/12 -- complete)

16 saints, plus a propers-slug update for the pre-existing Immaculate
Conception entry. Also extends the Advent temporal-category fix
(previous commit) to Christmastide: St. Thomas of Canterbury, only
Semiduplex, wins outright within the Christmas Octave in the real
Monastic 1617 engine, which the stronger privileged-feria tier would
have wrongly forbidden -- Christmastide now shares Advent's
privileged-feria-minor tier instead. Every real winner this month falls
within one of these two newly-corrected seasons.

This completes the first pass of the pre-1955 sanctoral calendar pull
across all twelve months.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/data/calendar/saints/holy-innocents.yml

@@ -0,0 +1,11 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live
+# kalendar check (2026-12-28) -- wins outright, Duplex II. classis, within
+# Christmas's Octave (privileged-feria-minor), Octave day commemorated in
+# return. About the Innocents slain at Christ's birth, not a feast of the
+# Lord/BVM directly, so kept in scope like every other real saint's day
+# this pull has found. Own proper collect and antiphon.
+id: holy-innocents
+name: "The Holy Innocents, Martyrs"
+rank: duplex-2-classis
+common: common-of-several-martyrs
+propers: "holy-innocents"