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-hermenegild.yml

@@ -0,0 +1,13 @@
+# Read directly from the reference engine: untagged/default [Rank]
+# block of `web/www/horas/Latin/Sancti/04-13.txt` -- "vide C2a;;Semiduplex;;2.2;;".
+# Own proper [Oratio]; no [Ant 1] of its own, so the Benedictus antiphon
+# falls back to Common of a Martyr (C2a), matching st-christopher.yml.
+# Nocturn 3 (Lectio7-9) is a direct `@Commune/C2:LectioN` cross-reference,
+# not transcribed here.
+id: st-hermenegild
+name: "St. Hermenegild, Martyr"
+rank: semiduplex
+common: common-of-a-martyr
+propers: "st-hermenegild"
+minorHoursCommon: common-of-a-martyr
+benedictusCommon: common-of-a-martyr