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-vitalis-collect.yml

@@ -0,0 +1,18 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617) — read
+# directly from the reference engine's own real-date output (2042-04-28).
+# Same convention as data/propers/temporal/*-collect.yml.
+id: st-vitalis-collect
+text:
+  la: |
+    Orémus.
+    Præsta, quǽsumus, omnípotens Deus: ut, qui beáti Vitális Mártyris tui natalítia cólimus, intercessióne ejus in tui nóminis amóre roborémur.
+    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.
+    Grant, we beseech thee, almighty God, that we who celebrate the martyrdom of blessed Vitalis, through his intercession be strengthened in your love.
+    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