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

tests/calendar/vigil-of-christmas-sunday.test.ts

@@ -0,0 +1,72 @@
+import { describe, expect, it } from 'vitest';
+import { resolveDay } from '../../src/calendar';
+import { resolveOrdo } from '../../src/hours';
+
+// Advent 4's own Sunday lands on Dec 24 (its latest possible date) roughly
+// every few years -- 2023 and 2028 are the two nearest examples. Per direct
+// instruction (2026-08): the Vigil of Christmas's own identity governs that
+// day (matching the reference engine's own rule -- Duplex I classis, never
+// displaced even by a privileged Sunday), with Advent 4 itself commemorated
+// alongside it -- a deliberate departure from the reference engine's own
+// `no commemoratio` for this exact collision, since the Sundays of Advent
+// have real standing of their own. See calendar/index.ts's
+// applyAdventFourVigilCommemoration and calendar/temporal-id.ts's own
+// `vigil-of-christmas` case.
+describe('Advent 4 Sunday landing on Dec 24 (the Vigil of Christmas)', () => {
+  it('2023-12-24: Vigil wins, Advent 4 commemorated', () => {
+    const day = resolveDay('2023-12-24');
+    expect(day.weekday).toBe('sunday');
+    expect(day.winner).toEqual({ kind: 'temporal', id: 'vigil-of-christmas' });
+    expect(day.commemorations).toEqual([{ kind: 'temporal', id: 'advent-4' }]);
+  });
+
+  it('2028-12-24: same shape, a second real occurrence', () => {
+    const day = resolveDay('2028-12-24');
+    expect(day.weekday).toBe('sunday');
+    expect(day.winner).toEqual({ kind: 'temporal', id: 'vigil-of-christmas' });
+    expect(day.commemorations).toEqual([{ kind: 'temporal', id: 'advent-4' }]);
+  });
+
+  it("Lauds carries both collects (Vigil's own, then Advent 4's commemorated) and the Vigil's own proper antiphons/Benedictus antiphon", () => {
+    const lauds = resolveOrdo('lauds', '2023-12-24');
+    const collects = lauds.parts.filter((p) => p.kind === 'prayer');
+    expect(collects).toHaveLength(2);
+    for (const c of collects) {
+      if (c.kind !== 'prayer') continue;
+      expect(c.text.status.la).toBe('verified');
+      expect(c.text.status.en).toBe('verified');
+    }
+    expect(collects[0]!.kind === 'prayer' ? collects[0]!.text.text.la : undefined).toContain(
+      'Deus, qui nos redemptiónis nostræ',
+    );
+    expect(collects[1]!.kind === 'prayer' ? collects[1]!.text.text.la : undefined).toContain(
+      'Excita, quǽsumus, Dómine',
+    );
+
+    const benedictus = lauds.parts.find((p) => p.kind === 'canticle' && p.canticleId === 'benedictus');
+    expect(benedictus?.kind).toBe('canticle');
+    if (benedictus?.kind === 'canticle') {
+      expect(benedictus.antiphon?.text.la).toContain('Oriétur');
+      expect(benedictus.antiphon?.status.la).toBe('verified');
+    }
+  });
+
+  it("Matins Nocturn 3 carries the Vigil's own Gospel (Matt 1:18-21) with St. Jerome's homily, still using the Sunday's fixed psalm table throughout", () => {
+    const matins = resolveOrdo('matins', '2023-12-24');
+    const gospels = matins.parts.filter((p) => p.kind === 'gospel');
+    expect(gospels).toHaveLength(1);
+    const gospel = gospels[0]!;
+    expect(gospel.kind).toBe('gospel');
+    if (gospel.kind === 'gospel') {
+      expect(gospel.text.citation?.la).toBe('Matt 1:18-21');
+      expect(gospel.text.status.la).toBe('verified');
+      expect(gospel.homily?.source).toContain('Jerome');
+      expect(gospel.homily?.text.status.la).toBe('verified');
+    }
+    // Nocturns 1-2 keep the ordinary Sunday psalm table (Ps 20-31),
+    // unaffected by which id won -- hours/matins.ts's threeNocturns/
+    // sundayPsalmNocturn path is keyed by day.weekday, not day.winner.id.
+    const psalmCount = matins.parts.filter((p) => p.kind === 'psalm').length;
+    expect(psalmCount).toBeGreaterThan(0);
+  });
+});