calendar: pull September's sanctoral calendar (pre-1955 pull, 9/12)

14 saints (12 real winners + 2 stubs), plus a propers-slug update for the
pre-existing Nativity of the BVM entry. Two monastic-calendar-specific
finds: St. Nicholas of Tolentino (Sep 10) overrides what the base
Tridentine table treats as a plain Nativity-octave continuation, and St.
Januarius (Sep 19) is a monastic-only addition absent from the base table
entirely. Also flags (not yet acted on) that the pre-existing Sep 29 St.
Michael mapping doesn't appear to hold in this pull's actual target
track -- see the sanctoral-calendar.yml comment for detail.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/data/calendar/saints/st-linus.yml

@@ -0,0 +1,9 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live
+# kalendar check (2025-09-23, 2026-09-23) -- winner outright, Semiduplex,
+# over the stub St. Thecla (see st-thecla.yml). No unique collect: the
+# raw file at this MM-DD slot (Sancti/09-23.txt) is empty.
+id: st-linus
+name: "St. Linus, Pope and Martyr"
+rank: semiduplex
+common: common-of-a-martyr-bishop
+propers: null