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-eusebius-of-vercelli.yml

@@ -0,0 +1,16 @@
+# A monastic-calendar-specific placement: the base Tridentine table
+# (1570.txt) doesn't have this saint at all; the monastic-specific delta
+# table (M1617.txt) places him here via a same-day redirect notation
+# ("12-15=12-16=..."). Verified live (Monastic Tridentinum 1617,
+# 2025-12-15) -- an ordinary Advent feria (privileged-feria-minor),
+# Semiduplex wins outright, feria commemorated in return. No unique
+# collect: the raw file actually found at this MM-DD slot
+# (Sancti/12-15.txt) is the later (post-1854) Octave of the Immaculate
+# Conception's, not this saint's at all -- a live prayTertia query for
+# the Monastic 1617 track returns no Oratio, confirming propers: null is
+# correct.
+id: st-eusebius-of-vercelli
+name: "St. Eusebius of Vercelli, Bishop and Martyr"
+rank: semiduplex
+common: common-of-a-martyr-bishop
+propers: null