Summary

• Adds an offline arrangement renderer using OfflineAudioContext and a small native PCM WAV encoder.
• Wires the sidebar Export button to render and download the full arrangement as stereo 44.1 kHz 16-bit PCM WAV.
• Includes arranged hybrid drums, Default Synth notes, Iowa Piano sampled notes, imported audio clips, track volume, track mute/solo, and master gain.
• Keeps export separate from the live audio engine so transport state, active voices, and live decoded caches are not mutated.
• Updates docs, feature status, Active Issue Order, and unit tests.

Changes

• Shared the existing PCM WAV decode fallback through src/audio/audio-buffer-decoder.ts.
• Added src/audio/offline-arrangement-renderer.ts for offline scheduling/rendering.
• Added src/audio/wav-encoder.ts for 16-bit PCM WAV Blob encoding.
• Extended arrangement audio events with optional audio clip duration/source offset metadata so export can crop imported audio placements.
• Added export busy/error UI in the project sidebar.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present 14 files / 93 tests
• npm run build --if-present
• Dev server smoke check: npm run dev -- --host 127.0.0.1 --port 5173; Vite used http://127.0.0.1:5174/, and curl -I returned HTTP 200.

Screenshots

Not captured. The UI change is the existing sidebar Export button gaining busy/error behavior.

Notes / Limitations

• Export renders the full arrangement length, not the current loop range.
• Imported audio clips are cropped to placement length; they are not time-stretched.
• No MP3/FLAC/stem export, mastering, normalization, or background worker rendering in this pass.

Closes #50