propers: add September's collect and antiphon text

6 collects, 1 antiphon -- including backfilling the pre-existing Nativity
of the BVM entry now that the pulling methodology exists.

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

src/data/propers/common/nativity-bvm-antiphon.yml

@@ -0,0 +1,12 @@
+# Verified against Divinum Officium (Latin/English Sancti/09-08.txt's own
+# [Ant 1] — the Lauds-hour antiphon, not Terce's. That file's own
+# "(rubrica cisterciensis)" alternate is a different text entirely; the
+# plain default is used here for consistency with every other saint file
+# in this set.
+id: nativity-bvm-antiphon
+text:
+  la: "Gloriósæ * Vírginis Maríæ ortum digníssimum recolámus, quæ et Genitrícis dignitátem obtínuit, et virginálem pudicítiam non amísit."
+  en: "Let us tell again * of the right worthy Birth of the glorious Virgin Mary, who gained the honour of Motherhood without losing the guilelessness of a Maid."
+status:
+  la: verified
+  en: verified