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-barbara.yml

@@ -0,0 +1,13 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live
+# kalendar check (2025-12-04) -- an ordinary Advent feria
+# (privileged-feria-minor), so this Simplex saint is only ever
+# commemorated here, the feria staying primary. No unique collect: the
+# raw file at this MM-DD slot (Sancti/12-04.txt) is actually St. Peter
+# Chrysologus's (a later Doctor-of-the-Church addition to this filename
+# slot) -- a live prayTertia query for the Monastic 1617 track returns no
+# Oratio at all, confirming propers: null is correct.
+id: st-barbara
+name: "St. Barbara, Virgin and Martyr"
+rank: simplex
+common: common-of-a-virgin-martyr
+propers: null