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/hours/resolve-common.ts

@@ -57,6 +57,7 @@ export const ALWAYS_OVERRIDE_TEMPORAL_IDS = new Set([
   'immaculate-heart-of-mary',
   'circumcision',
   'holy-name-of-jesus',
+  'vigil-of-christmas',
 ]);
 
 /**