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/st-sabina.yml

@@ -0,0 +1,10 @@
+# Genuinely unwinnable stub (verified 2025-08-29) -- always commemorated
+# under the Beheading of St. John the Baptist (see
+# decollation-of-st-john-baptist.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: st-sabina
+name: "St. Sabina, Martyr"
+rank: simplex
+common: common-of-a-martyr
+propers: null