Fix octave day labels: correct commemoration phrasing, closing-day title, and privileged-season precedence

Three related day-label bugs, all found via Aug 19's mislabeled Assumption
octave commemoration:

- A sanctoral winner that displaces the only active octave now shows the
  octave's real "Nth Day within the Octave of X" phrasing (no rank, since
  it's riding along under the winning feast), not a bare feast name.
- An octave's own closing day now titles itself "Octave of X", matching
  the real DO's "in octava" vs. "infra octavam" distinction, instead of
  "8th Day within the Octave of X".
- A sufficiently-ranked octave can now outrank a privileged temporal
  season (e.g. the Immaculate Conception's octave outright winning several
  of its days against Advent, live-verified against Divino Afflatu 1954),
  via a new shared octaveGoverningPrivilegedDay helper used by both the
  day label and the actual office-content resolver. Deliberately excludes
  Christmastide, whose own stacked octaves are structurally already that
  season's temporal content rather than a foreign add-on.

Updates a Matins test fixture that had unknowingly relied on the
Dec 15 bug (Advent ferial + a lone commemorated saint) and splits it into
a clean ferial case plus a dedicated commemorated-saint case.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/calendar/index.ts

@@ -356,10 +356,10 @@ export function isSundayOrFeast(day: LiturgicalDay): boolean {
   return day.weekday === 'sunday' || day.winner.kind === 'sanctoral';
 }
 
-export { compareFeastClass, isAtLeast, decideOccurrence } from './commemorations';
+export { compareFeastClass, isAtLeast, decideOccurrence, minimumOutrightWinningRank } from './commemorations';
 export { resolveCollision } from './collision';
 export { resolveTemporalId } from './temporal-id';
-export { activeOctavesFor, strictestThreshold, resolveActiveOctave } from './octaves';
+export { activeOctavesFor, strictestThreshold, resolveActiveOctave, octaveGoverningPrivilegedDay } from './octaves';
 export type { ActiveOctave } from './octaves';
 export type {
   LiturgicalDay,