calendar: real occurrence/precedence resolution (feast ranks, sanctoral content)

Replaces the FeastRank placeholder with a real ordered FeastClass (the old
pre-1955 six-level scale) and a new TemporalCategory (privileged/ordinary
Sunday/feria). calendar/commemorations.ts's decideOccurrence encodes the
actual precedence rules as explicit, commented branches per category
rather than the reference engine's own opaque numeric weights — this app
targets one ruleset, so the readability trade-off is worth it. Category
membership (data/calendar/temporal-categories.yml) is a best-effort
reconstruction of the pre-1955 tradition, not sourced from a primary text;
expect corrections.

calendar/feasts.ts resolves real sanctoral candidates, and resolveDay()
wires it all together — occurring/temporalCategory are no longer stubs.
isDoubleOrHigher (hours/antiphon.ts) is now a real comparison instead of
a hardcoded false.

Seeded 7 real saints/feasts, each verified directly against Divinum
Officium's own rank data (not reconstructed from memory) as a small,
growable start: St. Lawrence, the Assumption, St. Augustine, the Nativity
of the BVM, St. Michael, All Saints, the Immaculate Conception.

src/calendar/commemorations.ts

@@ -1,5 +1,75 @@
-// Precedence/occurrence rules for when a lower-ranked feast is commemorated
-// rather than fully displaced by the day's winning feast. Deliberately not
-// designed yet — see plan point 5 ("calendar rules are expected to iterate").
-// Unused until milestone 4.
-export {};
+// Precedence/occurrence resolution: given a day's temporal-cycle standing
+// (calendar/types.ts's TemporalCategory) and a candidate sanctoral feast (if
+// any), which one is actually kept, and whether the loser is commemorated.
+//
+// Modeled as explicit rules per category, not a numeric-weight comparison
+// like the reference engine's occurrence()/concurrence() — see
+// data/calendar/temporal-categories.yml's header for why. The thresholds
+// below are a best-effort reconstruction of the pre-1955 tradition (partly
+// grounded in General Rubrics §15/§16/§23, though that document is itself a
+// later, differently-numbered edition — see that file's header), not
+// verified against a primary source for this exact era. Expect corrections.
+
+import type { FeastClass, TemporalCategory } from './types';
+
+const FEAST_CLASS_ORDER: FeastClass[] = [
+  'simplex',
+  'semiduplex',
+  'duplex',
+  'duplex-majus',
+  'duplex-2-classis',
+  'duplex-1-classis',
+];
+
+export function compareFeastClass(a: FeastClass, b: FeastClass): number {
+  return FEAST_CLASS_ORDER.indexOf(a) - FEAST_CLASS_ORDER.indexOf(b);
+}
+
+export function isAtLeast(rank: FeastClass, threshold: FeastClass): boolean {
+  return compareFeastClass(rank, threshold) >= 0;
+}
+
+export interface OccurrenceResult {
+  winner: 'temporal' | 'sanctoral';
+  commemorated: boolean;
+}
+
+/**
+ * `sanctoral === null` means no feast is assigned to this date at all —
+ * temporal wins trivially, nothing to decide.
+ */
+export function decideOccurrence(temporal: TemporalCategory, sanctoral: FeastClass | null): OccurrenceResult {
+  if (!sanctoral) {
+    return { winner: 'temporal', commemorated: false };
+  }
+
+  switch (temporal) {
+    case 'ordinary-feria':
+      // An ordinary feria has no standing of its own to defend — any real
+      // feast, however low-ranked, is kept in its place.
+      return { winner: 'sanctoral', commemorated: false };
+
+    case 'privileged-feria':
+      // "These ferias are preferred to any feasts whatsoever, and they
+      // admit of no commemoration, except one of the privileged class"
+      // (General Rubrics §23) — read here as: only the very highest class
+      // even gets a mention.
+      return { winner: 'temporal', commemorated: isAtLeast(sanctoral, 'duplex-1-classis') };
+
+    case 'ordinary-sunday':
+      // An ordinary Sunday yields outright only to the highest class; a
+      // Double of the 2nd Class or a Greater Double is kept as a
+      // commemoration instead of displacing the Sunday; anything lower
+      // isn't even mentioned.
+      if (isAtLeast(sanctoral, 'duplex-1-classis')) {
+        return { winner: 'sanctoral', commemorated: false };
+      }
+      return { winner: 'temporal', commemorated: isAtLeast(sanctoral, 'duplex-majus') };
+
+    case 'privileged-sunday':
+      // "A Sunday of the 1st class is preferred to any feast whatsoever"
+      // (General Rubrics §15) — never displaced; commemorated only if the
+      // feast is otherwise of the very top ranks.
+      return { winner: 'temporal', commemorated: isAtLeast(sanctoral, 'duplex-2-classis') };
+  }
+}