Initial extraction: C++ MixerEngine standalone daemon

Extracted from op-pedal mixer-engine branch. Core components:

- MixerEngine / MixerChannelStrip / MixerBus — cleaned of LV2/Pedalboard/IHost/MidiMapper deps
- MixerControlServer — Unix domain socket JSON control interface
- main.cpp — JACK audio I/O client with headless fallback
- CMake build — auto-fetches nlohmann/json, links JACK
- 9 core tests passing

Architecture: JACK ports → MixerEngine::process() → JACK ports,
controlled via Unix socket JSON commands.

.gitignore

@@ -0,0 +1,13 @@
+build/
+*.o
+*.a
+*.so
+*~
+.DS_Store
+__pycache__/
+*.pyc
+.vscode/
+.idea/
+cmake-build-*/
+_deps/
+compile_commands.json