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/finding-of-st-stephen-antiphon.yml

@@ -0,0 +1,11 @@
+# Verified against Divinum Officium (Latin/English Sancti/08-03.txt's own
+# [Ant 1] — a deliberate, unmodified cross-reference to St. Stephen's own
+# Dec 26 antiphon. Duplicated content, not a shared reference, matching
+# this project's one-file-per-id convention.
+id: finding-of-st-stephen-antiphon
+text:
+  la: "Stéphanus autem * plenus grátia et fortitúdine, faciébat signa magna in pópulo."
+  en: "And Stephen, full of grace * and fortitude, did great signs among the people."
+status:
+  la: verified
+  en: verified