Generalize non-Sunday movable feasts into one Easter-offset resolver

The bespoke applyImmaculateHeart function (following the applyMarianSaturday/
applyChristTheKing precedent of one hand-written override function per
feast) doesn't scale: several more feasts are planned that all reduce to
"weekday N days from Easter Sunday" (Ember/Rogation days, more
Sacred-Heart-family Marian devotions, St. Joseph's pre-1955 Eastertide
feast, Lenten Friday Passion devotions), and one-off functions invite
subtle ordering bugs -- applyImmaculateHeart had to be sequenced after
applyMarianSaturday specifically or it would have been silently clobbered.

Replaced with calendar/movable-feasts.ts's generic applyMovableFeasts,
which scans calendar/temporal-feasts.ts's existing per-feast YAML records
for a new optional `easterOffset` field. Adding the next movable feast is
now a new data/calendar/temporal-feasts/<id>.yml file with `rank` and
`easterOffset` set, not a new TypeScript function. Same rank-compared,
commemorate-the-loser semantics as before, just centralized instead of
duplicated per feast.

christ-the-king, marian-saturday, and christmas-octave-sunday stay as
their own functions -- they're structurally different rules (a fixed
month-position search, a date-less fallback default, and a fixed-date-
range Sunday search, respectively), not Easter offsets, so force-fitting
them into this table wouldn't actually simplify anything.

No behavior change -- the existing Immaculate Heart of Mary test suite
passes unchanged through the new generic path.

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

src/calendar/movable-feasts.ts

@@ -0,0 +1,63 @@
+// A generic resolver for feasts pegged to a fixed day-offset from Easter
+// Sunday that AREN'T themselves a Sunday, so calendar/temporal-id.ts's own
+// governing-Sunday-based lookup can never find them -- e.g. Immaculate
+// Heart of Mary (Easter+69, always a Saturday). Every entry is just a
+// `data/calendar/temporal-feasts/<id>.yml` file with `rank` and
+// `easterOffset` set (temporal-feasts.ts's own `movableTemporalFeasts`
+// scans for these) -- adding the next one (an Ember/Rogation day, a
+// Sacred-Heart-family Marian feast, a Lenten Friday devotion, St.
+// Joseph's own Eastertide feast, ...) needs no new function here or in
+// calendar/index.ts.
+//
+// Deliberately doesn't try to cover every movable-date pattern in this
+// codebase: `christ-the-king` (last Sunday of October), `marian-saturday`
+// (a generic "nothing else assigned" fallback, not date-anchored at all),
+// and `christmas-octave-sunday` (whichever of Dec 26-29 is a Sunday) are
+// structurally different rules, not Easter-offset ones, and stay as their
+// own small functions in calendar/index.ts. A future Ember/Rogation day
+// anchored to a *fixed civil date's* nearest Sunday (September/Advent
+// Ember weeks, not Easter-anchored) would need a second anchor kind, not
+// modeled yet -- extend `TemporalFeastRecord`/this file's own resolver
+// when that's actually needed, rather than guessing at its shape now.
+import type { Commemoration, DayWinner } from './types';
+import { compareFeastClass } from './commemorations';
+import { easterSunday } from './easter';
+import { addDays, toIsoDate } from './date-math';
+import { movableTemporalFeasts } from './temporal-feasts';
+
+function dateForOffset(year: number, offset: number): string {
+  return addDays(toIsoDate(easterSunday(year)), offset);
+}
+
+/**
+ * Run last in resolveDay's override chain (see calendar/index.ts) so a
+ * specific named movable feast always supersedes whatever generic default
+ * (marian-saturday, a plain Sunday-of-week id) ran before it -- same
+ * reasoning as applyChristTheKing/applyMarianSaturday's own ordering.
+ * Same rank-compared, commemorate-the-loser shape those two already use:
+ * a temporal winner (no real standing of its own) is always superseded; a
+ * sanctoral winner keeps the day if its own rank is at least as strong as
+ * the movable feast's, which is commemorated instead; otherwise the
+ * movable feast wins and the displaced saint is commemorated.
+ */
+export function applyMovableFeasts(isoDate: string, winner: DayWinner, commemorations: Commemoration[]): DayWinner {
+  const year = Number(isoDate.slice(0, 4));
+  let resolvedWinner = winner;
+  for (const feast of movableTemporalFeasts()) {
+    if (isoDate !== dateForOffset(year, feast.easterOffset!)) {
+      continue;
+    }
+    const rank = feast.rank ?? 'simplex';
+    const FEAST_WINNER: DayWinner = { kind: 'temporal', id: feast.id };
+    if (resolvedWinner.kind === 'temporal') {
+      commemorations.push({ kind: 'temporal', id: resolvedWinner.id });
+      resolvedWinner = FEAST_WINNER;
+    } else if (compareFeastClass(resolvedWinner.rank, rank) >= 0) {
+      commemorations.push({ kind: 'temporal', id: feast.id });
+    } else {
+      commemorations.push({ kind: 'sanctoral', id: resolvedWinner.id, name: resolvedWinner.name, rank: resolvedWinner.rank });
+      resolvedWinner = FEAST_WINNER;
+    }
+  }
+  return resolvedWinner;
+}