feat: add MixerEngine core — ChannelStrip, MixerBus, and MixerEngine for band-in-a-box digital mixer

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.

src/CMakeLists.txt

@@ -369,6 +369,11 @@ set (PIPEDAL_SOURCES
     AudioDriver.hpp
     AudioConfig.hpp
 
+    # Mixer Engine (Band-in-a-Box)
+    MixerChannelStrip.cpp MixerChannelStrip.hpp
+    MixerBus.cpp MixerBus.hpp
+    MixerEngine.cpp MixerEngine.hpp
+
     ${VST3_SOURCES}
 )