calendar: occurrence engine v2 — real rank thresholds, transfers, collisions

Corrects and completes the occurrence rules, based on a design discussion
plus one concrete data point: St. Anthony Abbot (plain Duplex) was found
outright winning against an ordinary Sunday in the real Monastic 1617
engine, which the old duplex-1-classis-only threshold got wrong.

- FeastClass gains `vigil`, inserted between `simplex` and `semiduplex` —
  one ordering that correctly serves both "does this win against a Sunday"
  (vigil behaves like simplex there) and "which of two saints wins a
  landing-day collision" (vigil beats simplex, loses to semiduplex).
- LiturgicalDay.occurring (a flat OccurringFeast[] that could only ever
  express a losing *sanctoral* candidate) is replaced by `winner:
  DayWinner` + `commemorations: Commemoration[]` — a discriminated list
  that can hold the temporal day itself, one or more sanctoral entries, or
  (not built yet, but the shape already accommodates it) a future octave
  kind.
- commemorations.ts: ordinary Sundays let Duplex+ win outright (Sunday
  commemorated in return), Semiduplex/Vigil transfer elsewhere (too
  substantial a feast to cheapen with a bare commemoration), Simplex stays
  and is commemorated. Privileged Sundays never displace; Duplex-majus+
  commemorated, everything else transfers.
- collision.ts (new): resolves two sanctoral candidates wanting the same
  day (a transfer landing on an already-occupied day, or two native
  saints sharing a date) — duplex > semiduplex > vigil > simplex, loser
  always commemorated, ties favor the native occupant.
- temporal-id.ts (new): maps any date to one of the 52 real Sunday-collect
  ids from the previous commit, so a temporal winner/commemoration can
  actually be looked up, not just labeled "temporal" in the abstract.
- index.ts's resolveDay orchestrates all of it, including the actual
  Monday/Saturday transfer mechanism. Landing on a privileged feria (the
  concrete case: Holy Week, right after Palm Sunday) is explicitly
  deferred rather than guessed at — it needs its own Easter-keyed lookup
  table, the same way the reference engine handles it.

Added the Vigil of St. Lawrence (Aug 9) as real content specifically to
exercise the backward-transfer rule end-to-end: Aug 9, 2026 is a Sunday,
so the vigil transfers cleanly back to Saturday, verified by a new
integration test alongside the unit-level rule and collision tests.

tests/calendar/collision.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, it } from 'vitest';
+import { resolveCollision } from '../../src/calendar/collision';
+import type { SanctoralIdentity } from '../../src/calendar/types';
+
+function saint(id: string, rank: SanctoralIdentity['rank']): SanctoralIdentity {
+  return { id, name: id, rank };
+}
+
+describe('resolveCollision', () => {
+  it('higher rank wins regardless of which side is incoming vs native', () => {
+    const incoming = saint('incoming', 'semiduplex');
+    const native = saint('native', 'simplex');
+    const result = resolveCollision(incoming, native);
+    expect(result.winner).toEqual({ kind: 'sanctoral', id: 'incoming', name: 'incoming', rank: 'semiduplex' });
+    expect(result.commemorations).toEqual([{ kind: 'sanctoral', id: 'native', name: 'native', rank: 'simplex' }]);
+  });
+
+  it('a transferred Vigil beats a native Simplex (vigil ranks above simplex for collisions)', () => {
+    const result = resolveCollision(saint('vigil-feast', 'vigil'), saint('native', 'simplex'));
+    expect(result.winner.kind).toBe('sanctoral');
+    expect(result.winner.kind === 'sanctoral' ? result.winner.id : undefined).toBe('vigil-feast');
+  });
+
+  it('a native Semiduplex beats a transferred Vigil', () => {
+    const result = resolveCollision(saint('vigil-feast', 'vigil'), saint('native', 'semiduplex'));
+    expect(result.winner.kind === 'sanctoral' ? result.winner.id : undefined).toBe('native');
+    expect(result.commemorations).toEqual([{ kind: 'sanctoral', id: 'vigil-feast', name: 'vigil-feast', rank: 'vigil' }]);
+  });
+
+  it('ties favor the native occupant, the incoming feast is commemorated', () => {
+    const result = resolveCollision(saint('incoming', 'duplex'), saint('native', 'duplex'));
+    expect(result.winner.kind === 'sanctoral' ? result.winner.id : undefined).toBe('native');
+    expect(result.commemorations).toEqual([{ kind: 'sanctoral', id: 'incoming', name: 'incoming', rank: 'duplex' }]);
+  });
+
+  it('the loser is always commemorated, with no rank threshold', () => {
+    const result = resolveCollision(saint('incoming', 'duplex-1-classis'), saint('native', 'simplex'));
+    expect(result.commemorations).toEqual([{ kind: 'sanctoral', id: 'native', name: 'native', rank: 'simplex' }]);
+  });
+});