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-robert-collect.yml

@@ -0,0 +1,23 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617) — read
+# directly from the reference engine's own real-date output (2043-04-29).
+# Same convention as data/propers/temporal/*-collect.yml. One correction
+# to the source's own English translation: it left his name in the Latin
+# genitive ("blessed Robérti") rather than translating it — fixed to
+# "Robert" here as an evident transcription artifact, not a deliberate
+# stylistic choice (unlike the archaic English kept verbatim elsewhere in
+# this project).
+id: st-robert-collect
+text:
+  la: |
+    Orémus.
+    Intercéssio nos, quǽsumus, Dómine, beáti Robérti Abbátis comméndet: ut, quod nostris méritis non valémus, ejus patrocínio assequámur.
+    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.
+    May the pleading of blessed Robert, the Abbot, make us acceptable unto thee, O Lord, we pray; that what we may not have through any merits of ours, we may gain by means of his patronage.
+    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