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/day-label.test.ts

@@ -38,13 +38,14 @@ describe('getDayLabel — feast name combination', () => {
     weekday: 'monday',
     season: 'trinitytide',
     temporalCategory: 'ordinary-feria',
-    occurring: [],
+    winner: { kind: 'temporal', id: 'post-pentecost-02' },
+    commemorations: [],
   };
 
   it('shows just the feast name when it wins outright', () => {
     const day: LiturgicalDay = {
       ...base,
-      occurring: [{ id: 'x', name: 'St. Ereden', rank: 'duplex', commemorated: false }],
+      winner: { kind: 'sanctoral', id: 'x', name: 'St. Ereden', rank: 'duplex' },
     };
     expect(getDayLabel(day)).toBe('St. Ereden');
   });
@@ -52,12 +53,12 @@ describe('getDayLabel — feast name combination', () => {
   it('shows both, feast first, when the feast is merely commemorated', () => {
     const day: LiturgicalDay = {
       ...base,
-      occurring: [{ id: 'x', name: 'St. Ereden', rank: 'duplex-2-classis', commemorated: true }],
+      commemorations: [{ kind: 'sanctoral', id: 'x', name: 'St. Ereden', rank: 'duplex-2-classis' }],
     };
     expect(getDayLabel(day)).toBe('St. Ereden — Monday in the 2nd week after Trinity');
   });
 
-  it('shows just the temporal label when nothing is occurring at all', () => {
+  it('shows just the temporal label when nothing is commemorated at all', () => {
     expect(getDayLabel(base)).toBe('Monday in the 2nd week after Trinity');
   });
 });