Host/plugin crash at high midi_rate_hz (observed with IEM SceneRotator in REAPER)

[mpollow]

At high MIDI update rates, the host application can crash while CCTrack's
control changes drive plugin parameters. This was seen with one host and
one plugin, so the scope is unknown — it is recorded here rather than in
the docs, because we cannot generalise from a single combination.

Observed

• Setup: REAPER 7.79 (Linux, PipeWire), IEM SceneRotator 0.4.7 (VST3)
  reading CCTrack through the plugin's own MIDI input, output_mode=cc14,
  firmware v0.1.0.
• midi_rate_hz=200: REAPER aborts (SIGABRT), repeatedly, within minutes.
• midi_rate_hz=100 (the firmware default): no crash in testing.
• A pre-release firmware with a slower, uneven sample stream (~215 Hz instead
  of 400 Hz) crashed noticeably sooner at 200 Hz.

What the core dumps show

All dumps (coredumpctl info <pid>) show two threads inside the plugin's
parameter code at the same moment:

• audio thread: processBlock → setValueNotifyingHost → updateQuaternions
  → juce::AudioProcessorValueTreeState::ParameterAdapter::parameterValueChanged
• host main thread: setParamNormalized → SceneRotator::parameterChanged →
  updateEuler → the same ParameterAdapter code → abort()

Every CC is a parameter change that the plugin reports to the host, which the
host then echoes back; the Euler ↔ quaternion cross-updates run concurrently.
A higher MIDI rate means more parameter updates, and more opportunities for
the two paths to overlap.

Assessment

• The MIDI CCTrack sends is valid; the rate and timing only change how often
  the overlap occurs.
• Whether this is specific to this plugin, to this host, to VST3 parameter
  echo, or a more general pattern is untested. Only one plugin and one
  host have been tried, on Linux.

Mitigations for users

• Keep midi_rate_hz at the default 100, or lower it
  (tools/cctrack-cmd.py <port> set_config midi_rate_hz=50; takes effect
  immediately).
• Use the CDC quaternion stream (python/) where possible — it is unaffected.

Open

• Test more hosts/plugins and other plugin formats, on Linux, macOS and Windows.
• If the pattern holds up, report it upstream with the stack traces.