Adaptive soundtrack: urgent drums, crystal shimmer, flawless fanfare

[cmalpass]

Music now reacts to the run:

• Urgent drum layer (per-theme kick/hat patterns) crossfades in when hearts ≤ 2, a hazard looms within 420px ahead, or the Magma King is in the arena.
• Shimmer pad layer (lead +12 semitones, sparkle overtones) sparkles over crystal-dense stretches (4+ uncollected within 280px).
• Layers are always scheduled while music plays — only gains move — so transitions are click-free, and stopping music / victory fades them out.
• Flawless-run fanfare: a brighter 8-note arpeggio + bell shimmer plays when the victory comes with zero damage; plain run keeps the original fanfare.
• Pure trigger logic in new src/adaptive.ts (adaptiveFlags), wired through a new GameCtx.onPlayerHit hook and a hits counter in the game.

Validated locally (CI is failing for pre-existing unrelated reasons, so local validation is the gate):

• npm run typecheck ✓
• npm test — 209/209 (was 193; +9 pure adaptive tests, +7 game-level tests) ✓
• npm run build — 131.67 kB / 40.43 kB gzip ✓
• npm run lint ✓
• Headless Playwright E2E: audio unlocks, theme music starts, urgent gain crossfades to 0.885, shimmer to 0.839, urgent settles to 0.015, flawless victory routes {flawless: true}, post-hit victory routes {flawless: false}, zero page errors ✓

Screenshots: /tmp/tinyrex-e2e/01-urgent.png, 02-shimmer.png, 03-victory.png