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/hours/matins.ts

@@ -322,7 +322,7 @@ function ferialPsalmodyThreeNocturns(day: LiturgicalDay): [ResolvedPart[], Resol
 function nocturnReadingIds(day: LiturgicalDay, temporalId: string): string[] {
   const ids = new Set<string>();
   // Not gated to `kind === 'sanctoral'` -- a named temporal override (e.g.
-  // Immaculate Heart of Mary, calendar/index.ts's applyImmaculateHeart)
+  // Immaculate Heart of Mary, calendar/movable-feasts.ts's applyMovableFeasts)
   // has its own authored nocturn-readings file keyed by its own id too,
   // distinct from the plain governing-Sunday `temporalId` added below.
   // Harmless to include unconditionally: on an ordinary day `day.winner.id`