Generalize movable-feast anchors beyond Easter offsets, migrate Christ the King

Extends TemporalFeastRecord's date anchor from a single easterOffset
number into a MovableAnchor union: 'easter' (unchanged), plus
'nth-sunday-of-month' (nth: -1 for "last") and 'nth-sunday-of-advent' --
both new anchor kinds resolve to "Nth Sunday counting from an anchor
date," just with a different anchor (first-of-month vs. Advent's own
start, which is itself always a Sunday). Adds calendar/temporal.ts's
sundayOnOrAfter to support the month case.

Migrates christ-the-king.yml onto the new nth-sunday-of-month anchor
(month: 10, nth: -1) with a new `unconditional: true` flag replacing its
old always-wins-no-rank-check bespoke function -- retiring
applyChristTheKing and its lastSundayOfOctober helper from
calendar/index.ts entirely. Down to two bespoke override functions left
(marian-saturday's date-less fallback, christmas-octave-sunday's
fixed-date-range Sunday search), both structurally different from the
Nth-Sunday-from-anchor shape this table now covers.

No behavior change -- the full existing Christ the King 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/temporal-feasts.ts

@@ -8,24 +8,35 @@
 import type { FeastClass, OctaveConfig } from './types';
 import { easterOffsetOf } from './temporal';
 
+/** How to compute a movable feast's own date each year, for a feast that
+ * ISN'T itself a Sunday (or, for `unconditional` feasts like Christ the
+ * King, one whose occurrence rule is simpler to express as "always wins
+ * outright" than as an ordinary rank contest) -- see
+ * calendar/movable-feasts.ts, which is the only consumer of this.
+ * `resolveTemporalId`'s own Easter-offset resolution only ever finds one
+ * of the 52 canonical *Sunday* ids (it works by finding the governing
+ * Sunday on-or-before a date, then looking that Sunday's own offset up);
+ * none of these three anchor kinds are reachable through that lookup no
+ * matter what, so they need this separate mechanism instead. A feast
+ * that *is* a Sunday itself (Pentecost) doesn't need this at all -- it's
+ * already covered by the ordinary Sunday lookup, and by
+ * EASTER_OFFSET_STARTS below if it also has an octave to start. */
+export type MovableAnchor =
+  | { kind: 'easter'; offset: number }
+  | { kind: 'nth-sunday-of-month'; month: number; nth: number } // nth: 1-4, or -1 for "last"
+  | { kind: 'nth-sunday-of-advent'; nth: number }; // nth: 1-4 (Advent always has exactly 4 Sundays)
+
 export interface TemporalFeastRecord {
   id: string;
   name: string;
   octave?: OctaveConfig;
   rank?: FeastClass;
-  /** Days from Easter Sunday (0) this feast's own date falls on, for a
-   * feast that ISN'T itself a Sunday -- see calendar/movable-feasts.ts.
-   * `resolveTemporalId`'s own Easter-offset resolution only ever finds
-   * one of the 52 canonical *Sunday* ids (it works by finding the
-   * governing Sunday on-or-before a date, then looking that Sunday's own
-   * offset up); a weekday-anchored feast like Immaculate Heart of Mary
-   * (Easter+69, always a Saturday) is invisible to that lookup no matter
-   * what, so it needs this separate field plus calendar/movable-feasts.ts's
-   * own override pass instead. A feast that *is* a Sunday (Pentecost)
-   * doesn't need this at all -- it's already covered by the ordinary
-   * Sunday lookup, and by EASTER_OFFSET_STARTS below if it also has an
-   * octave to start. */
-  easterOffset?: number;
+  anchor?: MovableAnchor;
+  /** Christ the King's own rule: always wins the day outright, no rank
+   * contest at all (see calendar/movable-feasts.ts's applyMovableFeasts) --
+   * every other movable feast instead loses to an equal-or-higher-ranked
+   * sanctoral winner and is commemorated alongside it instead. */
+  unconditional?: boolean;
 }
 
 const temporalFeastModules = import.meta.glob<{ default: TemporalFeastRecord }>(
@@ -42,14 +53,14 @@ export function getTemporalFeastRecord(id: string): TemporalFeastRecord | undefi
   return temporalFeastsById.get(id);
 }
 
-/** Every temporal feast record carrying its own `easterOffset` -- the
- * table calendar/movable-feasts.ts's applyMovableFeasts scans. Adding a
- * new weekday-anchored movable feast (an Ember/Rogation day, a Sacred-
- * Heart-family Marian feast, a Lenten Friday devotion, ...) is just a new
- * `data/calendar/temporal-feasts/<id>.yml` file with `rank` and
- * `easterOffset` set -- no new code. */
+/** Every temporal feast record carrying its own `anchor` -- the table
+ * calendar/movable-feasts.ts's applyMovableFeasts scans. Adding a new
+ * movable feast (an Ember/Rogation day, a Sacred-Heart-family Marian
+ * feast, a Lenten Friday devotion, ...) is just a new
+ * `data/calendar/temporal-feasts/<id>.yml` file with `rank` and `anchor`
+ * set -- no new code. */
 export function movableTemporalFeasts(): TemporalFeastRecord[] {
-  return [...temporalFeastsById.values()].filter((f) => f.easterOffset !== undefined);
+  return [...temporalFeastsById.values()].filter((f) => f.anchor !== undefined);
 }
 
 /** Fixed-calendar-date starts (MM-DD -> temporal feast id). Only Christmas