feat: add pre/post fader toggle to aux sends

[ChuxiJ]

Summary

Currently, all sends are implicitly post-fader — the send taps audio after the channel volume/pan. Professional DAWs support pre-fader sends (tap before volume) for use cases like headphone mixes, parallel compression, and cue sends.

Reference: openDAW implements AudioSendRouting { Pre, Post } enum on AuxSendBox, with AuxSendProcessor applying ramped gains at the appropriate tap point.

Current State

// src/types/project.ts
interface Send {
  returnTrackId: string;
  amount: number; // 0-1
}

• Send always taps after volumeGain in TrackNode signal chain
• No option to choose tap point
• 2 hardcoded send slots (separate issue #TBD)

Proposed Changes

Data Model

interface Send {
  returnTrackId: string;
  amount: number; // 0-1
  preFader?: boolean; // default false (post-fader)
}

Audio Engine (TrackNode.ts)

• Create two tap points in signal chain:
  • Pre-fader tap: after EQ, before volumeGain
  • Post-fader tap: after volumeGain, before analyserNode
• Each send connects to the appropriate tap based on preFader flag
• Send gain node applies amount with 5ms ramp (click-free)

UI (MixerPanel.tsx)

• Add toggle button (PRE/POST) next to each send amount slider
• PRE = orange indicator, POST = default (matches DAW conventions)

Acceptance Criteria

• Send interface has preFader?: boolean field
• Pre-fader sends tap audio before volume/pan in TrackNode
• Post-fader sends tap audio after volume/pan (existing behavior, now explicit)
• Toggling pre/post updates audio routing in real-time without clicks
• UI shows PRE/POST toggle per send slot
• Undo/redo works for pre/post toggle changes
• Unit tests for pre/post signal path
• E2E test: toggle pre-fader, verify send level independent of fader

Files to Modify

• src/types/project.ts — Send interface
• src/engine/TrackNode.ts — Add pre/post tap points
• src/engine/AudioEngine.ts — Wire sends to correct tap
• src/store/projectStore.ts — updateTrackSend action
• src/components/mixer/MixerPanel.tsx — PRE/POST toggle UI