feat: add AudioContext sample playback

[britle20]

Summary

Closes #4.

Adds a browser Web Audio engine for loading bundled drum samples, decoding them into a runtime AudioBuffer cache, and triggering one-shot playback from a small typed UI-facing API.

Changes

• Added BrowserAudioEngine with AudioContext lifecycle, bundled sample fetch/decode, runtime buffer caching, and one-shot playback using fresh AudioBufferSourceNode instances.
• Added bundled sample metadata and four temporary synthetic wav samples for kick, snare, closed hi-hat, and open hi-hat.
• Added a minimal SamplePlaybackPanel for manual verification through user-triggered load/play actions.
• Added tests for bundled sample metadata.
• Updated docs and PLANS.md for AudioContext and Sample Playback #4.

Testing

• git diff --check
• file public/samples/drums/*.wav
• npm run typecheck
• npm run lint
• npm run test
• npm run build
• npm run dev -- --host 127.0.0.1
• curl -I http://127.0.0.1:5173/
• curl -I http://127.0.0.1:5173/samples/drums/kick.wav
• curl -I http://127.0.0.1:5173/samples/drums/snare.wav

Notes / Limitations

• Manual listening still needs browser review because this environment cannot confirm audible output.
• The four wav files are synthetic placeholders for this task only; future sample work can replace or expand them.
• No sequenced playback, lookahead scheduler, drum sequencer UI, piano roll UI, sample import, mixer, or effects are included.