calendar: pull April's sanctoral calendar (pre-1955 pull, 4/12)

12 saints for April (11 real + St. Catherine of Siena as a stub), all
winning outright on a clean per-annum date.

Two real methodological findings this month:
- St. Mark (Apr 25, Duplex II. classis) directly confirms the
  privileged-feria-major behavior from the previous fix: he only ever
  gets commemorated within the Easter Octave, never wins there.
- St. Robert/St. Catherine of Siena (Apr 29) is a genuine, year-stable
  monastic-calendar reassignment — this app's own base Latin source data
  files that date's Roman-calendar content under a different saint (Peter
  Martyr) entirely, which a naive raw-file lookup by MM-DD would have
  missed; the live per-date query resolved it correctly regardless, and
  is what these records are actually built from. Worth remembering for
  future months.

New common category: common-of-an-evangelist (St. Mark, an Evangelist
but not an Apostle).

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

src/data/calendar/saints/st-catherine-of-siena.yml

@@ -0,0 +1,13 @@
+# Secondary commemoration on 04-29 (loses the day to St. Robert, Abbot --
+# see st-robert.yml's note on this being a fixed monastic-calendar
+# reassignment, confirmed stable across two different years). Her real
+# historical/Roman-calendar date is Apr 30, but in this monastic calendar
+# specifically she sits on Apr 29 as Robert's secondary instead. No live
+# query exposes a secondary entry's own rank -- simplex here is an
+# unverified placeholder, same caveat as every other stub in this set.
+# No text pulled at all yet.
+id: st-catherine-of-siena
+name: "St. Catherine of Siena, Virgin"
+rank: simplex
+common: common-of-a-virgin
+propers: null