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>

src/calendar/index.ts

@@ -5,7 +5,8 @@ import { resolveTemporalId } from './temporal-id';
 import { getSanctoralCandidatesFor } from './feasts';
 import { decideOccurrence, isAtLeast, type OccurrenceResult } from './commemorations';
 import { resolveCollision } from './collision';
-import { addDays } from './date-math';
+import { addDays, toIsoDate } from './date-math';
+import { easterSunday } from './easter';
 import { activeOctavesFor, strictestThreshold } from './octaves';
 
 /**
@@ -104,10 +105,39 @@ export function resolveDay(isoDate: string): LiturgicalDay {
   winner = applyMarianSaturday(isoDate, weekday, temporalCategory, winner, commemorations);
   winner = applyChristTheKing(isoDate, winner, commemorations);
   winner = applyChristmasOctaveSunday(isoDate, weekday, winner, commemorations);
+  applyEpiphany6Commemoration(isoDate, commemorations);
 
   return { date: isoDate, weekday, season, temporalCategory, winner, commemorations };
 }
 
+/**
+ * Real rubric, confirmed 2026-08: when the 6th Sunday after Epiphany is
+ * bumped by Septuagesima — i.e. only in years where the 5th Sunday after
+ * Epiphany was the last one to actually occur — it's commemorated on the
+ * Saturday immediately before Septuagesima, on top of whatever else that
+ * Saturday already resolved to. Distinct from (and doesn't imply) the
+ * later "resumed post-Epiphany Sunday" transfer handled in
+ * temporal-id.ts's post-Pentecost overflow branch — that one applies to
+ * *every* skipped post-Epiphany Sunday, not just VI; this Saturday
+ * commemoration is VI's alone. Purely additive: no `Commemoration` variant
+ * needed beyond the existing `{ kind: 'temporal' }` one, since
+ * hours/resolve-common.ts's getDayCollects already resolves any temporal
+ * commemoration's own `${id}-collect` for free.
+ */
+function applyEpiphany6Commemoration(isoDate: string, commemorations: Commemoration[]): void {
+  const year = Number(isoDate.slice(0, 4));
+  const septuagesimaStart = addDays(toIsoDate(easterSunday(year)), -63);
+  const saturdayBeforeSeptuagesima = addDays(septuagesimaStart, -1);
+  if (isoDate !== saturdayBeforeSeptuagesima) {
+    return;
+  }
+  const lastEpiphanySunday = addDays(septuagesimaStart, -7);
+  if (resolveTemporalId(lastEpiphanySunday) !== 'post-epiphany-5') {
+    return;
+  }
+  commemorations.push({ kind: 'temporal', id: 'post-epiphany-6' });
+}
+
 /** The Sunday on or before Oct 31 — always lands in October since Oct 31
  * is at most 6 days after the month's last Sunday. */
 function lastSundayOfOctober(year: number): string {