Give the Vigil of Christmas (Dec 24) its own proper content

Non-Sunday Dec 24 previously just inherited Advent 4's ferial collect and
antiphons with nothing of its own. Import the Vigil's real proper content
from the reference engine (Duplex I classis): its own collect, all 5 Lauds
antiphons, the Benedictus antiphon, and Matins' Gospel (Matt 1:18-21) +
St. Jerome homily, wired via a new `vigil-of-christmas` temporal id scoped
to non-Sunday Dec 24 only. Advent IV landing on Dec 24 itself is untouched,
left as its own separate design question.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ME8QNPdEmHSbSx1r6VmDRG

src/data/propers/temporal/vigil-of-christmas-benedictus-antiphon.yml

@@ -0,0 +1,16 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), Lauds
+# live query, 2026-12-24 (a plain Thursday, non-Sunday Vigil of
+# Christmas). The static reference file (Sancti/12-24.txt) has no
+# explicit `[Ant Benedictus]` tag for this date -- turns out the live
+# engine reuses the same text tagged `[Ant 2]` there (labeled "Antiphona
+# ex Proprio Sanctorum" in the live render), already imported separately
+# for Matins' own Nocturn 2 -- see nocturn-readings/vigil-of-christmas.yml.
+# Same text, two real uses, not a coincidence to collapse into one file:
+# these are two distinct ResolvedPart slots.
+id: vigil-of-christmas-benedictus-antiphon
+text:
+  la: "Oriétur * sicut sol Salvátor mundi: et descéndet in úterum Vírginis, sicut imber super gramen, allelúja."
+  en: "The Saviour of the world * shall rise like the sun, and come down into the womb of the Virgin as the showers upon the grass, alleluia."
+status:
+  la: verified
+  en: verified