Muzica ambient T10: arpegiu calm care accelereaza sub 1 min

Opt-in din builder (checkbox music, default off). Orchestrator-only: parintele
detine AudioContext (reutilizeaza beep._ctx); camerele nu stiu de muzica.

- arpegiu pentatonica minora (oscilatoare sine scurte), tempo ~1.8x pe ultimul
  minut (legat de _deadline-ul Timer Calm); fara timer -> loop calm fara accelerare
- buton 🎵/🔇 in bara chrome (#btn-music)
- duck pe voce: voiceSay onstart/onend regleaza gain (vocea are prioritate)
- fallback fara AudioContext -> no-op, buton ascuns (zero penalizare)
- porneste la start + resume; stop la showFinale + toggle
- hook test window.__music; exemplu-campanie.html regenerat (ramane opt-in off)

Smoke 30/30 (test nou "muzica ambient": opt-in, start, tempo sub 1 min, duck, toggle).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -20,9 +20,9 @@ sursa de adevăr tehnică pentru agenți.
 python3 -m http.server 8000
 
 # Teste (Playwright; fără package.json commitat — vezi tests/AGENTS.md):
-npx playwright test tests/smoke.mjs                    # suita completă: 29/29
+npx playwright test tests/smoke.mjs                    # suita completă: 30/30
 npx playwright test tests/smoke.mjs --grep @regresie   # regresie: 16
-npx playwright test tests/smoke.mjs --grep @campanie   # campanie E2E: 15
+npx playwright test tests/smoke.mjs --grep @campanie   # campanie E2E: 16
 ```
 
 ## Durable Rules (repo-wide)