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.

src/calendar/day-label.ts

@@ -132,12 +132,11 @@ function temporalLabel(day: LiturgicalDay): string {
  * came from for the three cases.
  */
 export function getDayLabel(day: LiturgicalDay): string {
-  const winner = day.occurring.find((feast) => !feast.commemorated);
-  if (winner) {
-    return winner.name;
+  if (day.winner.kind === 'sanctoral') {
+    return day.winner.name;
   }
 
   const temporal = temporalLabel(day);
-  const commemorated = day.occurring.find((feast) => feast.commemorated);
+  const commemorated = day.commemorations.find((c) => c.kind === 'sanctoral');
   return commemorated ? `${commemorated.name} — ${temporal}` : temporal;
 }