feat(export): integrate 'Export Multitrack to encoderCraft' mixdown and stem rendering #1276

Summary

Add an "Export Multitrack to encoderCraft..." action to SoundCraft (parity with Adobe Audition's "Multitrack > Export to Adobe Media Encoder..."). This offloads multi-format audio encoding, batch stem splitting, and broadcast loudness normalization to encoderCraft.

Workflow & UI Requirements

1. Menu Item:
   • Add File > Export > Multitrack Mixdown to encoderCraft....
   • Add stem export option: "Queue Stems in encoderCraft".
2. IPC Integration:
   • Connect to local encoderCraft daemon (/tmp/encodercraft-$UID.sock or 127.0.0.1:8998).
3. Job Payload:
   • Submit JSON-RPC payload with:
     • Session file path or direct audio buffer stream.
     • Channel layout (Mono, Stereo, 5.1, 7.1).
     • Target loudness specifications (EBU R128 / ITU-R BS.1770-4 target LUFS, True Peak ceiling).
     • Discrete stem export configuration (individual stems with EBU BWF chunk headers).

Tracking & Cross-Links

• Target Parity: Adobe Audition -> Adobe Media Encoder batch queue
• encoderCraft Master Tracking Issue: el-j/encoderCraft#1
• encoderCraft Repository: el-j/encoderCraft
• Shared Dispatch Protocol: storytold/craft-libs#505
• Shared Media Codecs: storytold/craft-libs#503 / storytold/craft-libs#504
• SoundCraft Audio Migration PR: storytold/soundcraft#1275