calendar: pull August's sanctoral calendar (pre-1955 pull, 8/12)

18 saints (14 real winners + 4 stubs), plus propers-slug updates for the
4 pre-existing August entries. Notable this month: several dates have a
real named secondary saint permanently subordinate to an unmodeled fixed
octave (Lawrence's own, the Assumption's) or the fixed Transfiguration --
excluded entirely rather than guessed at, since this app can't yet
represent the actual blocking primary (a known, deferred octave gap).
Most notable casualty: St. Clare (Aug 12).

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

src/data/calendar/saints/decollation-of-st-john-baptist.yml

@@ -0,0 +1,13 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live
+# kalendar check (2025-08-29) -- winner outright, Duplex, over the stub
+# St. Sabina (see st-sabina.yml). About St. John the Baptist's beheading,
+# not a feast of the Lord/BVM, so kept in scope like his Nativity in June.
+# "common-of-an-apostle" is a loose fit (he isn't one) but the closest
+# existing category to "a major male saint honored with his own high
+# feast" -- same judgment call as nativity-of-st-john-the-baptist.yml.
+# Own proper collect and antiphon.
+id: decollation-of-st-john-baptist
+name: "The Beheading of St. John the Baptist"
+rank: duplex
+common: common-of-an-apostle
+propers: "decollation-of-st-john-baptist"