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/ss-twelve-brothers.yml

@@ -0,0 +1,9 @@
+# Genuinely unwinnable stub (verified 2025-09-01, 2026-09-01) -- always
+# commemorated under St. Giles (see st-giles.yml), never wins outright;
+# both are fixed, non-movable dates, so this isn't an artifact of the
+# tested year. rank/propers unverified placeholders.
+id: ss-twelve-brothers
+name: "Ss. Twelve Brothers, Martyrs"
+rank: simplex
+common: common-of-several-martyrs
+propers: null