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/immaculate-conception.yml

@@ -1,7 +1,9 @@
 # Verified against Divinum Officium (web/www/horas/Latin/Sancti/12-08.txt,
-# untagged/default [Rank] block).
+# untagged/default [Rank] block). Propers backfilled during the December
+# pull now that the collect-pulling methodology exists -- own proper
+# collect and antiphon.
 id: immaculate-conception
 name: "The Immaculate Conception of the Blessed Virgin Mary"
 rank: duplex-1-classis
 common: common-of-the-bvm
-propers: null
+propers: "immaculate-conception"