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-robert.yml

@@ -0,0 +1,17 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live kalendar
+# check (2029-04-29 and again 2043-04-29, confirming this is a fixed
+# monastic-calendar reassignment, not year-dependent like St. Francis of
+# Paola's transfer) -- winner outright, Duplex, over St. Catherine of
+# Siena (see st-catherine-of-siena.yml). Own proper collect, no unique
+# antiphon of his own. Genuine methodological note: this app's base Latin
+# source data files the Roman-calendar content for this date (04-29.txt)
+# under St. Peter Martyr instead -- Robert's own real content lives
+# somewhere the monastic-specific delta correctly resolves to, which the
+# live-engine query reaches even though a naive raw-file lookup by MM-DD
+# wouldn't have. Worth remembering if a future month's raw-file check
+# ever seems to contradict what a live query actually shows.
+id: st-robert
+name: "St. Robert, Abbot"
+rank: duplex
+common: common-of-an-abbot
+propers: "st-robert"