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-francis-of-paola.yml

@@ -0,0 +1,11 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live kalendar
+# check (2042-04-14) -- his native date is Apr 2, but Holy Week blocked him
+# that year and he transferred here instead, winning outright against
+# Ss. Tiburtius/Valerian/Maximus (his own file's own real content, so still
+# reliably read even though the date isn't his usual one) -- Duplex. Own
+# proper collect, no unique antiphon of his own.
+id: st-francis-of-paola
+name: "St. Francis of Paola, Confessor"
+rank: duplex
+common: common-of-a-confessor
+propers: "st-francis-of-paola"