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/st-nicholas.yml

@@ -0,0 +1,12 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live
+# kalendar check (2025-12-06) -- an ordinary Advent feria
+# (privileged-feria-minor); Semiduplex is enough to win outright here
+# (unlike the stricter privileged-feria/Ember tier), with the feria
+# itself commemorated in return -- the concrete case that revealed this
+# month's temporal-category fix. Own proper collect, no unique antiphon
+# of his own.
+id: st-nicholas
+name: "St. Nicholas, Bishop and Confessor"
+rank: semiduplex
+common: common-of-a-confessor-bishop
+propers: "st-nicholas"