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-subbuild/CMakeLists.txt

@@ -0,0 +1,42 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file LICENSE.rst or https://cmake.org/licensing for details.
+
+cmake_minimum_required(VERSION 4.3.4)
+
+# Reject any attempt to use a toolchain file. We must not use one because
+# we could be downloading it here. If the CMAKE_TOOLCHAIN_FILE environment
+# variable is set, the cache variable will have been initialized from it.
+unset(CMAKE_TOOLCHAIN_FILE CACHE)
+unset(ENV{CMAKE_TOOLCHAIN_FILE})
+
+# We name the project and the target for the ExternalProject_Add() call
+# to something that will highlight to the user what we are working on if
+# something goes wrong and an error message is produced.
+
+project(nlohmann_json-populate NONE)
+
+
+# Pass through things we've already detected in the main project to avoid
+# paying the cost of redetecting them again in ExternalProject_Add()
+set(GIT_EXECUTABLE [==[/usr/bin/git]==])
+set(Git_VERSION [==[2.53.0]==])
+set_property(GLOBAL PROPERTY _CMAKE_FindGit_GIT_EXECUTABLE_VERSION
+  [==[/usr/bin/git;2.53.0]==]
+)
+
+
+include(ExternalProject)
+ExternalProject_Add(nlohmann_json-populate
+                     "UPDATE_DISCONNECTED" "False" "GIT_REPOSITORY" "https://github.com/nlohmann/json.git" "EXTERNALPROJECT_INTERNAL_ARGUMENT_SEPARATOR" "GIT_TAG" "v3.11.3"
+                    SOURCE_DIR          "/home/oplabs/projects/oplabs-mixer-daemon/build/_deps/nlohmann_json-src"
+                    BINARY_DIR          "/home/oplabs/projects/oplabs-mixer-daemon/build/_deps/nlohmann_json-build"
+                    CONFIGURE_COMMAND   ""
+                    BUILD_COMMAND       ""
+                    INSTALL_COMMAND     ""
+                    TEST_COMMAND        ""
+                    USES_TERMINAL_DOWNLOAD  YES
+                    USES_TERMINAL_UPDATE    YES
+                    USES_TERMINAL_PATCH     YES
+)
+
+