feat(audio): sound effects, BGM, volume and procedural synth

[xinyao27]

Summary

Adds a complete audio feature to PocketJS as a new host surface globalThis.audio (spec ops 26–31) — deliberately outside the deterministic core, mirroring the DevTools-ops contract so goldens and input tapes are untouched byte-for-byte.

• Public API (@pocketjs/framework/audio, AUDIO.md): playSfx / defineSfx / playSynth (8-bit synth descriptors: square/pulse/triangle/saw/sine/noise + sweep + envelope), playBgm / pauseBgm / resumeBgm / stopBgm (loop, cross-fade), setVolume / setMuted (master/sfx/bgm buses).
• Asset pipeline: <appDir>/sounds.json bakes WAVs → mono s16 SND pak entries (audio:sfx.* / audio:bgm.*), resampled at build time so hosts do zero decoding; BGM budget warning at 1 MB/track.
• Web host: WebAudio graph (master→sfx/bgm buses), gesture unlock (deferred BGM, dropped SFX pre-unlock), synth descriptors pre-rendered into cached AudioBuffers.
• PSP host: one hardware channel + an allocation-free, integer-only (Q15/16.16) software mixer thread (priority 16), fixed 8-voice pool with oldest-of-kind stealing, SPSC command ring, cross-fade via a borrowed dying voice; PCM plays straight from .rodata.
• Sim host: records the audio command stream into Trace.audio; the music-demo journey asserts ops land on exact frames, chaos-equal.
• Demos: music plays 3 generated chiptune loops (deterministic gen-sounds.ts) with cross-fading track switches and a synth blip; settings' SOUND EFFECTS toggle is now real.
• Docs: new AUDIO.md (spec + usage), touchpoints in README/DESIGN/RUNTIMES.

Verification

• bun run test — 168 + 9 + 4 pass, 0 fail (incl. new audio.test.ts ×22, sound-bake.test.ts ×20, music sim journey)
• bun run golden — 35/35 unchanged; bun run tape:check — 180 frames match (audio provably outside the replay world)
• bunx tsc --noEmit clean; music / settings / hero builds green
• Review fixes applied on top of the implementation: PCM cursor widened to u64 (BGM > 65535 frames), click-free fade rounding, stop-while-paused zombie voice

Not verified here

The PSP native mixer (native/src/audio.rs) compiles against the vendored rust-psp checkout, which is absent on this machine — needs a cargo check + PPSSPP/hardware smoke in a PSP-toolchain environment before ready-for-review.