Resolve Advent IV landing on Dec 24 as the Vigil, with Advent IV commemorated

When Advent 4's own Sunday lands on Dec 24, the Vigil of Christmas's
identity (rank, collect, Lauds antiphons, Matins Gospel nocturn) now
governs the day, matching the reference engine's own rule that a
privileged Sunday never displaces it. Unlike the reference engine's own
"no commemoratio" for this exact collision, Advent 4 is deliberately
still commemorated — the Sundays of Advent have real standing of their
own. Matins' three-nocturn Sunday psalmody is untouched either way,
since it's driven by weekday, not by which id wins.

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

src/calendar/temporal-id.ts

@@ -30,7 +30,6 @@
 // confirmed against that source rather than re-derived by hand.
 import { resolveSeason, sundayOnOrBefore, firstSundayStrictlyAfter, adventStart, easterOffsetOf } from './temporal';
 import { daysBetween } from './date-math';
-import { weekdayOf } from './weekday';
 
 const EASTER_OFFSET_IDS: [number, string][] = [
   [-63, 'septuagesima'],
@@ -84,13 +83,16 @@ export function resolveTemporalId(isoDate: string): string {
   // Advent 4's own — see data/propers/temporal/vigil-of-christmas-collect.yml
   // and data/propers/nocturn-readings/vigil-of-christmas.yml, both
   // transcribed directly from the reference engine's Sancti/12-24.txt.
-  // Scoped to non-Sunday only for now: when Dec 24 itself is Advent 4's own
-  // Sunday, the reference engine's own 12-24s.txt keeps Advent 4's psalms
-  // and most of its Matins lessons (only swapping in the Vigil's Gospel
-  // nocturn) rather than fully substituting this id — a genuine content
-  // splice this app doesn't model yet, deliberately left as `advent-4` for
-  // that one specific case pending that design decision.
-  if (isoDate.slice(5) === '12-24' && weekdayOf(isoDate) !== 'sunday') {
+  // Applies even when Dec 24 itself is Advent 4's own Sunday — per direct
+  // instruction (2026-08), the Vigil's own identity (rank/collect/Lauds
+  // antiphons) governs that day too, with Advent 4 folded in as a
+  // commemoration instead (calendar/index.ts's
+  // applyAdventFourVigilCommemoration) rather than the reference engine's
+  // own `no commemoratio` rule. Matins' three-nocturn Sunday psalmody is
+  // unaffected either way — it's driven by `day.weekday`, not by this id
+  // (hours/matins.ts's own `threeNocturns` check), so Advent 4's "Psalmi
+  // Dominica" standing survives this id swap for free.
+  if (isoDate.slice(5) === '12-24') {
     return 'vigil-of-christmas';
   }