propers: add April's collect text

Real collect text for all 8 April saints that have one — same extraction
method as prior months. One correction to St. Robert's own English
translation in the source, which left his name in Latin genitive form
("blessed Robérti") rather than translating it — fixed, flagged in the
file as a transcription artifact, not a deliberate style choice.

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

src/data/propers/common/st-leo-i-collect.yml

@@ -0,0 +1,18 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617) — read
+# directly from the reference engine's own real-date output (2043-04-11).
+# Same convention as data/propers/temporal/*-collect.yml.
+id: st-leo-i-collect
+text:
+  la: |
+    Orémus.
+    Exáudi, quǽsumus, Dómine, preces nostras, quas in beáti Leónis Confessóris tui atque Pontíficis solemnitáte deférimus: et, qui tibi digne méruit famulári, ejus intercedéntibus méritis, ab ómnibus nos absólve peccátis.
+    Per Dóminum nostrum Jesum Christum, Fílium tuum: qui tecum vivit et regnat in unitáte Spíritus Sancti, Deus, per ómnia sǽcula sæculórum.
+    ℟. Amen.
+  en: |
+    Let us pray.
+    Hear, O Lord, we beseech thee, the prayers which we offer thee on this the solemn feast-day of thy blessed Confessor and bishop Leo, and, for the sake of him who so nobly served thee, forgive us our trespasses.
+    Through Jesus Christ, thy Son our Lord, Who liveth and reigneth with thee, in the unity of the Holy Ghost, God, world without end.
+    ℟. Amen.
+status:
+  la: verified
+  en: verified