Arrangement WAV export

[britle20]

Feature Spec

Related feature document: docs/features/21-arrangement-wav-export.md

Goal

Render the current arrangement to a downloadable WAV file.

Context

WAV is the first export target because it can be generated directly from rendered PCM without adding a codec dependency. Export timing should come from audio engine/offline scheduling, not UI timers.

Scope

Included:

• Export action for the current arrangement.
• Offline rendering of the full arrangement length.
• WAV Blob encoding, initially stereo 44.1 kHz 16-bit PCM unless implementation constraints justify another format.
• Include arranged clips and mixer routing available at implementation time.
• Busy/progress/error UI.

Excluded:

• MP3/FLAC/AAC.
• Stem export.
• Realtime recording.
• Mastering/normalization.
• Cloud upload.

Constraints

• Do not mutate live playback state while exporting.
• Keep offline rendering separate from React components.
• Do not add an encoder dependency unless the PR justifies it.

Done when

• Export downloads a playable WAV file.
• WAV duration matches the arrangement length.
• Missing sample data produces a clear error.
• Live playback still works after export completes or fails.

Verification

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Notes

Planning created from the agentic workflow discuss-feature process.

[britle20]

Implemented by PR #58, merged into develop.