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/saints/st-anselm.yml

@@ -0,0 +1,18 @@
+# Read directly from the reference engine: untagged/default [Rank]
+# block of `web/www/horas/Latin/Sancti/04-21.txt` -- "vide C4a;;Duplex;;3;;".
+# No [Oratio]/[Ant 1] of its own in this file at all -- both fall back
+# to Common of a Confessor Bishop who is a Doctor (C4a), matching
+# st-ambrose.yml/st-isidore-of-seville.yml. Nocturn 3 (Lectio7-9) is a
+# direct `@Commune/C4a:LectioN in 2 loco` cross-reference, not
+# transcribed here.
+id: st-anselm
+name: "St. Anselm, Bishop, Confessor and Doctor of the Church"
+rank: duplex
+common: common-of-a-confessor-bishop
+propers: null
+collectCommon: collect-c4a
+collectName:
+  la: ["Ansélmum"]
+  en: ["Anselm"]
+minorHoursCommon: common-of-a-confessor-bishop
+benedictusCommon: common-of-a-confessor-bishop