Add 6 more April saints previously never added

St. Isidore of Seville, St. Vincent Ferrer, St. Hermenegild, St. Anselm,
St. Fidelis of Sigmaringen, St. Peter Canisius -- April complete.
St. Peter Canisius is the second common-of-a-confessor-doctor saint
(after St. John Damascene in March), same benedictusCommon reuse
pattern. St. Hugh, Abbot (Apr 1) checked and correctly skipped: his own
file is Commemoratio-rank with only a collect, no Lectio content at
all to source.

npm test (365 tests) and tsc --noEmit both pass.

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

src/data/calendar/sanctoral-calendar.yml

@@ -90,12 +90,18 @@ days:
   # transfers due to Holy Week; that's the existing transfer machinery,
   # not something recorded specially here.
   "04-02": [st-francis-of-paola]
+  "04-04": [st-isidore-of-seville]
+  "04-05": [st-vincent-ferrer]
   "04-11": [st-leo-i]
+  "04-13": [st-hermenegild]
   "04-14": [ss-tiburtius-valerian-and-maximus]
   "04-17": [st-anicetus]
+  "04-21": [st-anselm]
   "04-22": [ss-soter-and-caius]
   "04-23": [st-george]
+  "04-24": [st-fidelis-of-sigmaringen]
   "04-25": [st-mark]
+  "04-27": [st-peter-canisius]
   "04-26": [ss-cletus-and-marcellinus]
   "04-28": [st-vitalis]
   "04-29": [st-robert, st-catherine-of-siena]