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/vigil-of-st-matthew.yml

@@ -0,0 +1,17 @@
+# A real vigil. Sep 20 sometimes coincides with the movable September
+# Ember Saturday (privileged-feria, already modeled -- which specific
+# week in September Ember falls in shifts year to year, confirmed by
+# comparing 2025, where Sep 20 IS Ember Saturday, against 2026, where
+# Sep 20 is an ordinary Sunday instead). In the *real* engine, on an
+# Ember-Saturday year this vigil is commemorated Mass-only rather than
+# fully displaced (2025-09-20, live check) -- this app doesn't model that
+# Office-vs-Mass distinction anywhere, so here it simply transfers per the
+# ordinary privileged-feria/vigil rule instead, a known, already-accepted
+# simplification (not specific to this saint) rather than a guess. No
+# unique collect: the raw file at this MM-DD slot (Sancti/09-20.txt) is
+# empty.
+id: vigil-of-st-matthew
+name: "Vigil of St. Matthew"
+rank: vigil
+common: common-of-a-vigil
+propers: null