MixerEngine architecture:
- MixerChannelStrip: per-input FX chain (Lv2Pedalboard reuse),
volume, pan, mute, solo, HPF, aux sends, VU metering
- MixerBus: accumulation bus with volume, mute, VU. Supports
master, subgroup, aux, and FX-return bus types
- MixerEngine: orchestrator managing channel→bus routing graph,
bus→bus routing, solo override, and the full real-time audio
processing cycle
All new code compiles cleanly with the existing C++20 build and
follows the existing PiPedal codebase conventions (namespaces,
error handling, buffer patterns). CPU-efficient real-time thread
processing with atomic control surface interaction.
shawn