Implement the resumed-post-Epiphany-Sunday temporal-id algorithm

Post-Pentecost overflow years (early Easter, fewer than 6 Sundays after
Epiphany fit before Septuagesima) previously clamped every excess Sunday
to post-pentecost-24 instead of resuming the skipped post-Epiphany
Sundays' own content, per the traditional rubric. Ported directly from
the reference engine's own DivinumOfficium/Date.pm getweek().

Also adds the Epiphany VI Saturday-before-Septuagesima commemoration
(only when Epiphany V was the last Sunday to actually occur), and fixes
day-label.ts's on-screen display to track the same resumed-Sunday
content instead of a raw elapsed-week count — including a correction so
the fixed final Sunday of the year always reads "23rd Sunday after
Trinity", not a per-year-varying number.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/calendar/temporal-id.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, it } from 'vitest';
+import { resolveTemporalId } from '../../src/calendar/temporal-id';
+import { resolveDay } from '../../src/calendar';
+
+describe('resolveTemporalId — post-Pentecost overflow', () => {
+  // 2026: Easter 2026-04-05 -> only post-epiphany-1..5 occur before
+  // Septuagesima, so post-pentecost-24 through 27 (5 real weeks between
+  // XXIII and the last Sunday) resume as Epiphany V and VI before landing
+  // on the fixed final Sunday. Confirmed against the reference engine's
+  // own DivinumOfficium/Date.pm getweek() (Pent/Epi arithmetic), not just
+  // hand-derived.
+  it('resumes the skipped post-Epiphany Sundays in order before the fixed last Sunday', () => {
+    expect(resolveTemporalId('2026-11-07')).toBe('post-pentecost-23');
+    expect(resolveTemporalId('2026-11-14')).toBe('post-epiphany-5');
+    expect(resolveTemporalId('2026-11-21')).toBe('post-epiphany-6');
+    expect(resolveTemporalId('2026-11-28')).toBe('post-pentecost-24');
+  });
+
+  it('does not resume anything in a non-overflow year (all 6 post-Epiphany Sundays occurred)', () => {
+    // 1943: Easter 1943-04-25, latest possible-ish Easter -> all 6
+    // post-Epiphany Sundays occur, so post-pentecost climbs past 24
+    // without ever touching Epiphany content, capping at the fixed id.
+    expect(resolveTemporalId('1943-11-21')).toBe('post-pentecost-24');
+  });
+});
+
+describe('post-Epiphany VI commemoration on the Saturday before Septuagesima', () => {
+  it('fires only when Epiphany V was the last Sunday to occur (2022, 2025)', () => {
+    const day2022 = resolveDay('2022-02-12');
+    expect(day2022.commemorations).toContainEqual({ kind: 'temporal', id: 'post-epiphany-6' });
+
+    const day2025 = resolveDay('2025-02-15');
+    expect(day2025.commemorations).toContainEqual({ kind: 'temporal', id: 'post-epiphany-6' });
+  });
+
+  it('does not fire when all 6 post-Epiphany Sundays occurred (1943)', () => {
+    const day = resolveDay('1943-02-20');
+    expect(day.commemorations).not.toContainEqual({ kind: 'temporal', id: 'post-epiphany-6' });
+  });
+});