Surround mixing, VST3 hosting, and hardening against hostile inputs

Surround:
- main and bus formats up to 9.1.6 and Ambisonics; pairwise constant-power
  surround panner (divergence, centre, LFE, height) per track and send;
  ITU fold-down; multichannel bounce with correct WAV channel masks;
  multichannel device output; surround panner and multichannel meters in the
  Mix window; setup.main_format, mix.surround_pan, mix.send_surround_pan

VST3:
- soundcraft-vst3-host: isolated unsafe crate (vst3 bindings, MIT), scans
  standard folders, hosts components/processors/controllers, params,
  latency, notes; VST3 menu; engine.vst3_plugins; e2e tests with a fixture

Hardening:
- positions from parameters clamped; offline renders capped and bounded to
  track material; loop/transport limits; hostile-parameter property test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xtask/src/main.rs

@@ -23,6 +23,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("midi", 0),
     ("dsp", 1),
     ("clap-host", 1),
+    ("vst3-host", 1),
     ("model", 2),
     ("mix", 3),
     ("engine", 4),