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

tests/hours/matins.test.ts

@@ -371,3 +371,39 @@ describe('resolveOrdo("matins", ...) Sacred Triduum', () => {
     expect(ordo.parts[1]).toMatchObject({ kind: 'psalm', psalmNumber: 3 });
   });
 });
+
+describe('resolveOrdo("matins", ...) Assumption-octave hymn fallback (2026-08-22 fix)', () => {
+  // Real bug: resolveMatinsHymn's own doc comment already claimed the
+  // Assumption's octave "keeps the feast's own proper hymn all week," but
+  // the code only ever matched when the day's own winner literally *was*
+  // `assumption` (Aug 15 itself) -- every other octave day fell straight
+  // through to the plain ferial hymn instead. Fixed by adding a real
+  // octave-fallback tier (resolveActiveOctave -> `matins-hymn-${octave.id}`).
+  function hymnLatin(date: string): string | undefined {
+    const ordo = resolveOrdo('matins', date);
+    const hymn = ordo.parts.find((p) => p.kind === 'hymn') as { text: { text: { la: string } } } | undefined;
+    return hymn?.text.text.la;
+  }
+
+  it("St. Bernard's own day (Aug 20, no hymn of his own authored) picks up the Assumption's octave hymn, not the plain ferial one", () => {
+    expect(hymnLatin('2026-08-20')).toContain('Surge');
+  });
+
+  it('every other interior day governed by the Assumption octave (16, 19, 21 — none with a hymn of their own) also picks up the octave hymn', () => {
+    for (const date of ['2026-08-16', '2026-08-19', '2026-08-21']) {
+      expect(hymnLatin(date)).toContain('Surge');
+    }
+  });
+
+  it("Aug 17 is instead governed by St. Lawrence's own octave (its own elevated closing day, outranking the Assumption's ordinary day here) — no hymn authored for it, so it still falls to the plain ferial one, correctly", () => {
+    expect(hymnLatin('2026-08-17')).toContain('Somno');
+  });
+
+  it('Aug 22, the octave-closing day (Immaculate Heart of Mary relocated away), also gets the Assumption hymn via the pre-existing sanctoral-substitution path', () => {
+    expect(hymnLatin('2026-08-22')).toContain('Surge');
+  });
+
+  it("a plain ferial day outside the octave still gets the plain ferial hymn, unaffected", () => {
+    expect(hymnLatin(FERIAL_DATE)).toContain('Somno');
+  });
+});