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-damasus.yml

@@ -0,0 +1,11 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live
+# kalendar check (2025-12-11) -- an ordinary Advent feria
+# (privileged-feria-minor), Semiduplex wins outright, feria commemorated
+# in return. Own proper collect (Latin only -- see
+# propers/common/st-damasus-collect.yml for the missing-English note), no
+# unique antiphon of his own.
+id: st-damasus
+name: "St. Damasus I, Pope and Confessor"
+rank: semiduplex
+common: common-of-a-confessor-bishop
+propers: "st-damasus"