feat: scaffold project framework with engine, node skeleton, and passthrough node

- CMake build system with vcpkg dependency management
- libdmf-node: Node interface, port definitions, WebSocket control
  server, node runner with MXL lifecycle management
- libdmf-engine: Graph model (nodes/edges CRUD, serialization),
  FlowManager (UUID + V210 NMOS flow definitions), ProcessManager
  (fork/exec node processes), ApiServer (REST API for graph control)
- Passthrough node: reads V210 grains from MXL, copies, writes to MXL
- Unit tests for Graph and FlowManager (6 cases, 21 assertions)
- MXL SDK as git submodule (symlinked from extern/mxl)

CMakeLists.txt

@@ -0,0 +1,31 @@
+cmake_minimum_required(VERSION 3.24 FATAL_ERROR)
+
+project(dmf-studio
+    VERSION 0.1.0
+    LANGUAGES CXX C
+)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+
+option(DMF_BUILD_TESTS "Build tests" ON)
+option(DMF_BUILD_MXL_TOOLS "Build MXL tools (testsrc, sink)" OFF)
+
+find_package(fmt CONFIG REQUIRED)
+find_package(spdlog CONFIG REQUIRED)
+find_package(nlohmann_json CONFIG REQUIRED)
+find_package(Libwebsockets CONFIG REQUIRED)
+find_package(Catch2 CONFIG QUIET)
+
+add_subdirectory(extern/mxl)
+add_subdirectory(libs/dmf-node)
+add_subdirectory(libs/dmf-engine)
+add_subdirectory(nodes/passthrough)
+add_subdirectory(engine)
+
+if(DMF_BUILD_TESTS AND Catch2_FOUND)
+    add_subdirectory(tests)
+endif()