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-fidelis-of-sigmaringen.yml

@@ -0,0 +1,12 @@
+# Read directly from the reference engine: untagged/default [Rank]
+# block of `web/www/horas/Latin/Sancti/04-24.txt` -- "vide C2a;;Duplex;;3;;".
+# Own proper [Oratio]; no [Ant 1] of its own, so the Benedictus antiphon
+# falls back to Common of a Martyr (C2a), matching st-hermenegild.yml.
+# No Lectio7-9 in the source (6-lesson office).
+id: st-fidelis-of-sigmaringen
+name: "St. Fidelis of Sigmaringen, Martyr"
+rank: duplex
+common: common-of-a-martyr
+propers: "st-fidelis-of-sigmaringen"
+minorHoursCommon: common-of-a-martyr
+benedictusCommon: common-of-a-martyr