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/ss-tiburtius-valerian-and-maximus.yml

@@ -0,0 +1,9 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live kalendar
+# check (2043-04-14, a year St. Francis of Paola doesn't transfer here) --
+# winner outright, Simplex. Own proper collect, no unique antiphon of
+# their own.
+id: ss-tiburtius-valerian-and-maximus
+name: "Ss. Tiburtius, Valerian, and Maximus, Martyrs"
+rank: simplex
+common: common-of-several-martyrs
+propers: "ss-tiburtius-valerian-and-maximus"