feat: Apply effects chain in WAV/stem export via OfflineAudioContext

[ChuxiJ]

Problem

WAV export (exportMix.ts) renders the mix without applying any track effects. Users who spend time crafting their sound with reverb, compression, EQ etc. get a completely dry export that sounds nothing like what they heard during playback.

Proposed Solution

1. Rebuild the full signal chain (source → effects → sends → master) in an OfflineAudioContext
2. Extract effect chain factory from TrackNode to share between live and offline contexts
3. Apply master chain (EQ, compressor, limiter) in offline render
4. Support both full mix export and per-stem export with effects

Technical Notes

• Reference: TASK_QUEUE.md item S1-04
• Depends on: Effects connected to live audio path (sibling issue)
• Key files: src/services/exportMix.ts, src/engine/TrackNode.ts

Acceptance Criteria

• WAV export includes all track effects
• WAV export includes master chain processing
• Stem export renders each track with its effects
• Exported audio matches playback output (within floating-point tolerance)
• Export progress indicator works correctly
• Unit test: compare exported buffer has different RMS than dry source
• No memory leaks from OfflineAudioContext disposal

[ChuxiJ]

Closing as already implemented. buildOfflineEffects() in src/engine/exportMix.ts (line 279) builds native Web Audio effect chains for offline rendering. exportMixToWav() and renderMixOffline() both apply track effects via OfflineAudioContext. Supported: eq3, parametricEq, compressor, distortion, filter, delay. Reverb is intentionally skipped (needs IRs for offline).

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