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.

build/CMakeFiles/mixer.dir/cmake_clean.cmake

@@ -0,0 +1,15 @@
+file(REMOVE_RECURSE
+  "CMakeFiles/mixer.dir/src/MixerBus.cpp.o"
+  "CMakeFiles/mixer.dir/src/MixerBus.cpp.o.d"
+  "CMakeFiles/mixer.dir/src/MixerChannelStrip.cpp.o"
+  "CMakeFiles/mixer.dir/src/MixerChannelStrip.cpp.o.d"
+  "CMakeFiles/mixer.dir/src/MixerEngine.cpp.o"
+  "CMakeFiles/mixer.dir/src/MixerEngine.cpp.o.d"
+  "libmixer.a"
+  "libmixer.pdb"
+)
+
+# Per-language clean rules from dependency scanning.
+foreach(lang CXX)
+  include(CMakeFiles/mixer.dir/cmake_clean_${lang}.cmake OPTIONAL)
+endforeach()