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-test.dir/flags.make

@@ -0,0 +1,10 @@
+# CMAKE generated file: DO NOT EDIT!
+# Generated by "Unix Makefiles" Generator, CMake Version 4.3
+
+# compile CXX with /usr/bin/c++
+CXX_DEFINES = 
+
+CXX_INCLUDES = -I/home/oplabs/projects/oplabs-mixer-daemon/include -I/home/oplabs/projects/oplabs-mixer-daemon/build/_deps/nlohmann_json-src/include
+
+CXX_FLAGS = -std=gnu++17 -fPIE -Wall -Wextra -Wpedantic
+