Add Ember-day mechanism (privileged feria, not a rank contest) + real content

Researched whether September/Advent Ember days belong in the
movable-feasts.ts table alongside Christ the King/Immaculate Heart of
Mary. Live-verified against the reference engine they don't: Ember days
are privileged ferias (a saint can still win outright against them), not
named feasts contesting the day by rank.

Two smaller pieces instead: calendar/temporal.ts's septemberEmberDayOffset
(Sunday nearest Sept 14, same "nearest" arithmetic adventStart already
used, factored into a shared nearestSunday helper) feeds a new
resolveTemporalCategory check giving those 3 dates privileged-feria-minor
(live-verified correct); calendar/ember-days.ts's applyEmberDay relabels
the day's own temporal id to the Ember day's own (so its real content is
found) when the feria itself wins, or adds a commemoration when a saint
does. Advent Ember days needed no precedence change -- Advent's own
season default already covers them.

Found and fixed a real bug along the way: matins.ts's nocturnReadingIds
only ever pulled a commemorated *sanctoral* id's own readings, never a
commemorated *temporal* one's -- so a commemorated Ember day (the common
case) would never have surfaced its own content. Same root cause as the
day.winner.id gap the IHM relocation fixed earlier, just hiding in the
commemorations loop instead.

Authored all 6 days' real collect + 3 Matins readings each, transcribed
directly from the reference engine (September: Tempora/093-{3,5,6}.txt;
Advent: Tempora/Adv3-{3,5,6}.txt, whose lessons are themselves a
cross-reference to the Annunciation's own Common, followed and
transcribed from there). Kept as 3 separate readings per day rather than
the usual combine-into-one default, since each carries its own
genuinely distinct proper responsory.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VZSAgRi4QE4XRTqVto93zA

src/calendar/temporal.ts

@@ -42,12 +42,21 @@ function fixedDateFor(id: string): { month: number; day: number } {
 const CHRISTMAS = fixedDateFor('christmas-day');
 const EPIPHANY = fixedDateFor('epiphany');
 
+/** The Sunday nearest `isoDate` — ties (exactly 3-4 days either way can't
+ * happen; a week has one middle point, Wednesday, which is always closer
+ * to one side or the other) resolved the usual "nearest" way: on or
+ * before if `isoDate` is Sun-Wed, the next one if Thu-Sat. Shared by
+ * `adventStart` (nearest Nov 30) and calendar/ember-days.ts's own
+ * September Ember anchor (nearest Sept 14). */
+export function nearestSunday(isoDate: string): string {
+  const dow = dayOfWeek(isoDate);
+  const delta = dow <= 3 ? -dow : 7 - dow;
+  return addDays(isoDate, delta);
+}
+
 /** The Sunday nearest Nov 30 (St. Andrew's Day) — Advent's real start rule. */
 export function adventStart(year: number): string {
-  const nov30 = `${year}-11-30`;
-  const dow = dayOfWeek(nov30);
-  const delta = dow <= 3 ? -dow : 7 - dow;
-  return addDays(nov30, delta);
+  return nearestSunday(`${year}-11-30`);
 }
 
 /**
@@ -147,6 +156,41 @@ export function isInTriduum(isoDate: string): boolean {
   return offset >= -3 && offset <= -1;
 }
 
+const SEPTEMBER_EMBER_OFFSETS = [3, 5, 6]; // Wed, Fri, Sat after the anchor Sunday
+const ADVENT_EMBER_OFFSETS = [3, 5, 6]; // Wed, Fri, Sat after the 3rd Sunday of Advent
+
+/** The Sunday nearest Sept 14 (Exaltation of the Holy Cross) — anchors
+ * September's Ember days, same "nearest" rule as Advent's own start.
+ * Live-verified (2026, 2019) against the reference engine: genuinely
+ * "nearest," not "3rd Sunday of the calendar month" — those two
+ * computations disagree in some years (e.g. 2025). */
+export function septemberEmberAnchor(year: number): string {
+  return nearestSunday(`${year}-09-14`);
+}
+
+/** Which of September's 3 Ember-day offsets (3/5/6 = Wed/Fri/Sat)
+ * `isoDate` is, if any — shared by resolveTemporalCategory below (the
+ * precedence side: these get `privileged-feria-minor`) and
+ * calendar/ember-days.ts (the content-id side: each gets its own proper
+ * collect/readings instead of inheriting the governing Sunday's). */
+export function septemberEmberDayOffset(isoDate: string): number | undefined {
+  const year = Number(isoDate.slice(0, 4));
+  const anchor = septemberEmberAnchor(year);
+  return SEPTEMBER_EMBER_OFFSETS.find((offset) => addDays(anchor, offset) === isoDate);
+}
+
+/** Which of Advent's 3 Ember-day offsets `isoDate` is, if any. Unlike
+ * September's, this doesn't need its own resolveTemporalCategory entry —
+ * Advent's ordinary `privileged-feria-minor` season default already
+ * covers it (no live evidence found that Advent Ember days need a
+ * stronger tier) — but it still needs its own content id, same as
+ * September's, hence still exported for calendar/ember-days.ts. */
+export function adventEmberDayOffset(isoDate: string): number | undefined {
+  const year = Number(isoDate.slice(0, 4));
+  const thirdAdventSunday = addDays(adventStart(year), 14);
+  return ADVENT_EMBER_OFFSETS.find((offset) => addDays(thirdAdventSunday, offset) === isoDate);
+}
+
 /**
  * A day's precedence category under the temporal cycle alone — see
  * calendar/types.ts's TemporalCategory doc comment and
@@ -172,6 +216,15 @@ export function resolveTemporalCategory(isoDate: string, season: Season, weekday
     return fixedOverride;
   }
 
+  // September Ember days: live-verified privileged-feria-minor (Ss.
+  // Cornelius & Cyprian, Semiduplex, won outright there with the feria
+  // demoted to a commemoration) — see septemberEmberDayOffset's own doc
+  // comment for why this can't be expressed as a plain Easter offset or
+  // fixed MM-DD the way every other override in this function is.
+  if (septemberEmberDayOffset(isoDate) !== undefined) {
+    return 'privileged-feria-minor';
+  }
+
   const offset = easterOffsetOf(isoDate);
 
   const singleOverride = temporalCategories.offsets?.[String(offset)];