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/data/calendar/saints/vigil-of-st-lawrence.yml

@@ -0,0 +1,10 @@
+# A real vigil, used as the concrete first test case for the vigil rank
+# and its backward-transfer rule (see calendar/commemorations.ts and
+# calendar/index.ts) — vigils are the day *before* their feast (St.
+# Lawrence is Aug 10, so this is Aug 9), and if that date is impeded (e.g.
+# falls on a Sunday), it transfers to the day before instead of forward.
+id: vigil-of-st-lawrence
+name: "Vigil of St. Lawrence"
+rank: vigil
+common: common-of-a-vigil
+propers: null

src/data/calendar/sanctoral-calendar.yml

@@ -12,6 +12,7 @@
 # privileged-day lists. `example-confessor` is deliberately NOT mapped to
 # any day here — it's fictional, kept only to document the record shape.
 days:
+  "08-09": [vigil-of-st-lawrence]
   "08-10": [st-lawrence]
   "08-15": [assumption]
   "08-28": [st-augustine]