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/ss-tiburtius-valerian-and-maximus-collect.yml

@@ -0,0 +1,18 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617) — read
+# directly from the reference engine's own real-date output (2043-04-14).
+# Same convention as data/propers/temporal/*-collect.yml.
+id: ss-tiburtius-valerian-and-maximus-collect
+text:
+  la: |
+    Orémus.
+    Præsta, quǽsumus, omnípotens Deus: ut, qui sanctórum Mártyrum tuórum Tibúrtii, Valeriáni et Máximi sollémnia cólimus; eórum étiam virtútes imitémur.
+    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.
+    Almighty God, for Whom holy Tiburtius, Valerian, and Maximus uplifted their testimony, grant, we beseech Thee, unto all whosoever call the same to solemn memory, grace to imitate the example of their godly courage.
+    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