Protect external plugin bus buffers and loading

[gijzelaerr]

Summary

• Allocate scratch channels for every enabled plugin bus during instrument warm-up and MIDI rendering. Match that channel coverage in audio processing. This addresses the out-of-bounds access described in Segmentation fault (usually) when loading Spectrasonics VST3 plugins #346.
• Skip parameter probing during load_plugin() when no initial parameter values were supplied. This addresses the load-time hang described in load_plugin() parameter sweep floods The Usual Suspects instruments with SysEx and makes them hang #513.
• Add a regression test that detects parameter probing during a default load.

Validation

• Native editable build and full test suite passed locally.
• Python lint, formatting, type checking, C++ formatting, and git diff --check passed locally.

The affected third-party instruments are not bundled as test fixtures. Keep #346 and #513 open until their reporters verify the change with those instruments.