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/_deps/nlohmann_json-build/nlohmann_jsonConfigVersion.cmake

@@ -0,0 +1,20 @@
+# This is essentially cmake's BasicConfigVersion-SameMajorVersion.cmake.in but
+# without the 32/64-bit check.  Since json is a header-only library, it doesn't
+# matter if it was built on a different platform than what it is used on (see
+# https://github.com/nlohmann/json/issues/1697).
+set(PACKAGE_VERSION "3.11.3")
+
+if(PACKAGE_VERSION VERSION_LESS PACKAGE_FIND_VERSION)
+  set(PACKAGE_VERSION_COMPATIBLE FALSE)
+else()
+
+  if(PACKAGE_FIND_VERSION_MAJOR STREQUAL "3")
+    set(PACKAGE_VERSION_COMPATIBLE TRUE)
+  else()
+    set(PACKAGE_VERSION_COMPATIBLE FALSE)
+  endif()
+
+  if(PACKAGE_FIND_VERSION STREQUAL PACKAGE_VERSION)
+      set(PACKAGE_VERSION_EXACT TRUE)
+  endif()
+endif()