Add Dec 26-30's own Sunday rule -- a deliberate remix, not a live-verified fix

Per direct instruction, and explicitly framed as this project's own
departure from Monastic 1617 itself: within Dec 26-30 inclusive (exactly
one real Sunday falls somewhere in that window every year), whichever
date is the actual Sunday always wins as Nat1-0 outright, with no rank
check at all -- even Stephen/John/Holy Innocents (Duplex II. classis)
get fully displaced, not just commemorated. The displaced saint isn't
commemorated in place either; he reappears in full as Dec 30's own
winner instead (the block's one date with no fixed saint of its own).
Rank never matters for this rule -- confirmed with Becket (Semiduplex)
transferring the same way as the Duplex-II-classis saints.

This deliberately diverges both from what Monastic 1617 itself does on
Dec 26-29 (live-verified two commits ago: the saint wins there, Sunday
merely commemorated) and from the general privileged-sunday transfer
rule elsewhere (which only transfers ranks below duplex-majus and
commemorates everything else in place) -- this window transfers every
rank and commemorates nothing in place, a genuinely new, third
behavior specific to this 5-day block.

Implemented as calendar/index.ts's applyChristmasOctaveSunday, a
narrowly-scoped layer in the same spirit as applyMarianSaturday/
applyChristTheKing, rather than by changing the shared
decideOccurrence/transfer machinery every other Sunday relies on. Dec 31
and everything outside Dec 26-30 is untouched -- St. Silvester still
beats the Sunday there via the plain, unchanged ordinary-sunday rule.

Verified against all five possible Sunday positions within the window
(Dec 26/27/28/29/30) plus the "nothing displaces anything" control case,
each with its own dedicated test in the new
tests/calendar/christmas-octave-sunday.test.ts. Updates two existing
tests whose assertions were correct under the old rule and are now
superseded specifically within this window (Stephen/Innocents no longer
win their own Sunday-collision dates there) while confirming the
underlying ordinary-sunday rule is still live and real outside it (Dec
31/St. Silvester).

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

tests/hours/lauds.test.ts

@@ -141,7 +141,11 @@ describe('resolveOrdo("lauds", ...)', () => {
   });
 
   it('resolves a real Benedictus antiphon for christmas-octave-sunday too, pulled from the reference engine\'s own dedicated Tempora/Nat1-0 proper rather than any individual octave-day saint\'s page', () => {
-    const ordo = resolveOrdo('lauds', '2026-12-30');
+    // 2029-12-30 specifically: the year Dec 30 itself is the real Sunday
+    // within the Octave, so nothing gets bumped there by
+    // calendar/index.ts's applyChristmasOctaveSunday (see its own tests) --
+    // a genuinely clean case for this content, unlike most years.
+    const ordo = resolveOrdo('lauds', '2029-12-30');
     const canticle = ordo.parts.find((p) => p.kind === 'canticle' && p.canticleId === 'benedictus');
     expect(canticle?.kind === 'canticle' ? canticle.antiphon?.text.la : undefined).toContain('Dum médium siléntium');
     expect(canticle?.kind === 'canticle' ? canticle.antiphon?.status?.la : undefined).toBe('verified');