propers: add August's collect and antiphon text

11 collects, 4 antiphons -- including backfilling the three pre-existing
August entries (Lawrence, Assumption, Augustine) now that the pulling
methodology exists. Also updates three pre-existing tests whose dates
collided with this month's new real content: the vigil-of-St.-Lawrence
backward-transfer case now runs into a real feast at its landing day
instead of an empty one (confirmed correct against the live reference
engine, including the surviving commemoration on the impeded Sunday
itself), and two other tests moved off Aug 9/10 to dates unaffected by
the new data.

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

src/data/propers/common/decollation-of-st-john-baptist-collect.yml

@@ -0,0 +1,17 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617) — read
+# directly from the reference engine's own real-date output (2025-08-29).
+id: decollation-of-st-john-baptist-collect
+text:
+  la: |
+    Orémus.
+    Sancti Joánnis Baptístæ Præcursóris et Mártyris tui, quǽsumus, Dómine, veneránda festívitas: salutáris auxílii nobis præstet efféctum:
+    Qui vivis et regnas cum Deo Patre, in unitáte Spíritus Sancti, Deus, per ómnia sǽcula sæculórum.
+    ℟. Amen.
+  en: |
+    Let us pray.
+    Lord, we beseech thee, that the keeping of this honourable feastday in memory of the holy Baptist John, thy Fore-runner, and thy Martyr, may be a mean to draw upon us the effectual outpouring of thy saving help.
+    Who livest and reignest with God the Father in the unity of the Holy Spirit, God, world without end.
+    ℟. Amen.
+status:
+  la: verified
+  en: verified