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/sanctoral-calendar.yml

@@ -67,6 +67,22 @@ days:
   "03-12": [st-gregory-the-great]
   "03-19": [st-joseph]
   "03-21": [st-benedict]
+  # April — 4/12 of the pre-1955 pull. St. Robert/St. Catherine of Siena
+  # (Apr 29) is a genuine monastic-calendar reassignment, not a mistake —
+  # see st-robert.yml. St. Francis of Paola's native date (Apr 2) often
+  # transfers due to Holy Week; that's the existing transfer machinery,
+  # not something recorded specially here.
+  "04-02": [st-francis-of-paola]
+  "04-11": [st-leo-i]
+  "04-14": [ss-tiburtius-valerian-and-maximus]
+  "04-17": [st-anicetus]
+  "04-22": [ss-soter-and-caius]
+  "04-23": [st-george]
+  "04-25": [st-mark]
+  "04-26": [ss-cletus-and-marcellinus]
+  "04-28": [st-vitalis]
+  "04-29": [st-robert, st-catherine-of-siena]
+  "04-30": [st-peter-martyr]
   "08-09": [vigil-of-st-lawrence]
   "08-10": [st-lawrence]
   "08-15": [assumption]