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-collect.yml

@@ -0,0 +1,17 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617) — read
+# directly from the reference engine's own real-date output (2025-09-08).
+id: nativity-bvm-collect
+text:
+  la: |
+    Orémus.
+    Fámulis tuis, quǽsumus, Dómine, cæléstis grátiæ munus impertíre: ut, quibus beátæ Vírginis partus éxstitit salútis exórdium; Nativitátis ejus votíva solémnitas pacis tríbuat increméntum.
+    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 unto us thy servants, we beseech thee, O Lord, the gift of thy heavenly grace, unto whom Thou didst give the first sight of a Saviour as the offspring of a Blessed Virgin, and grant that this Feast, which they keep in honour of the same Virgin, may avail them unto the increase of peace.
+    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