Fix Matins hymn to actually keep the Assumption's octave hymn all week

resolveMatinsHymn's own doc comment already claimed the octave "keeps
the feast's own proper hymn all week," but the code never implemented
it -- it only matched when the day's own winner literally was
`assumption` (Aug 15 itself). Every other octave day (16, 17, 19, 20,
21, none of whom has a Matins hymn of their own authored) fell straight
through to the plain ferial hymn instead.

Adds a real octave-fallback tier between the existing per-feast override
and the seasonal tier. A deliberate departure from the reference engine,
which doesn't do this either (Monastic 1617's octave days have no
[Hymnus Matutinum] override at all) -- consistent with how this project
already treats octave readings/commemorations more generously than any
one source track.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VZSAgRi4QE4XRTqVto93zA

src/hours/matins.ts

@@ -62,7 +62,7 @@
 // full-calendar content pass. See TODO.md for what's deferred.
 import type { ResolvedOrdo, ResolvedPart, ResolvedText, ResolvedVerse } from './types';
 import type { LiturgicalDay } from '../calendar/types';
-import { resolveDay, resolveTemporalId, activeOctavesFor } from '../calendar';
+import { resolveDay, resolveTemporalId, activeOctavesFor, resolveActiveOctave } from '../calendar';
 import { isInTriduum } from '../calendar/temporal';
 import { getDayLabel } from '../calendar/day-label';
 import { getPsalmVerses } from '../psalter';
@@ -215,14 +215,28 @@ function sundayCanticleNocturn(group: SundayNocturn, day: LiturgicalDay): Resolv
 /** The Matins hymn — a duplex-majus+ saint's or eligible named temporal
  * feast's own proper hymn (`matins-hymn-${overrideId}`, via the same
  * getOfficeOverrideId eligibility Lauds/Vespers' own resolveOffice uses),
- * when authored, else falls to a *seasonal* default (Advent/Lent/
+ * when authored; else, on a day within an active octave whose own feast
+ * has a hymn authored, that octave's hymn (`matins-hymn-${octave.id}`) —
+ * see below; else falls to a *seasonal* default (Advent/Lent/
  * Passiontide/Paschaltide, none authored yet), else the plain year-round
- * ferial hymn — same override > season > ferial precedence every other
- * hour's own office bundle uses (see lauds.ts's resolveOffice), just never
- * wired up here before. Concrete motivating case (user, 2026-08-21): the
- * Assumption's octave (`Sancti/08-21bmv.txt`'s own `[Rule] ex Sancti/
- * 08-15`) genuinely keeps the feast's own proper hymn all week, not the
- * ferial one this always rendered previously. */
+ * ferial hymn — same override > octave > season > ferial precedence every
+ * other hour's own office bundle uses (see lauds.ts's resolveOffice), just
+ * never fully wired up here before.
+ *
+ * Concrete motivating case (user, 2026-08-21/22): the Assumption's octave
+ * (`Sancti/08-21bmv.txt`'s own `[Rule] ex Sancti/08-15`) genuinely keeps
+ * the feast's own proper hymn all week — a claim this comment already
+ * made before the octave tier below actually existed, which only ever
+ * fired on Aug 15 itself (the one day `overrideId` literally *is*
+ * `assumption`). Every other octave day (16, 17, 19, 20, 21, each with
+ * its own named saint who has no Matins hymn of their own authored) fell
+ * straight through to the plain ferial hymn instead. This is a deliberate
+ * departure from the reference engine, which doesn't do this either
+ * (Monastic 1617's own octave days have no `[Hymnus Matutinum]` override
+ * at all, live-checked 2026-08-22) — not a restoration of source
+ * behavior, just this project's own generous octave design (see
+ * "Not a reconstruction" in the repo's CLAUDE.md) applied to the hymn the
+ * same way it's already applied to readings/commemorations elsewhere. */
 function resolveMatinsHymn(day: LiturgicalDay): ResolvedText {
   const overrideId = getOfficeOverrideId(day);
   if (overrideId) {
@@ -231,6 +245,13 @@ function resolveMatinsHymn(day: LiturgicalDay): ResolvedText {
       return proper;
     }
   }
+  const octave = resolveActiveOctave(day.date);
+  if (octave) {
+    const octaveHymn = resolveCommon(`matins-hymn-${octave.id}`);
+    if (octaveHymn.status.la !== 'missing' || octaveHymn.status.en !== 'missing') {
+      return octaveHymn;
+    }
+  }
   const seasonSuffix = seasonalOfficeSuffix(day.season);
   if (seasonSuffix) {
     const seasonal = resolveCommon(`matins-hymn-${seasonSuffix}`);