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)

libs/dmf-engine/CMakeLists.txt

@@ -0,0 +1,18 @@
+add_library(dmf-engine STATIC
+    src/graph.cpp
+    src/flow_manager.cpp
+    src/process_manager.cpp
+    src/api_server.cpp
+)
+
+target_include_directories(dmf-engine PUBLIC
+    include
+)
+
+target_link_libraries(dmf-engine PUBLIC
+    dmf-node
+    nlohmann_json::nlohmann_json
+    spdlog::spdlog
+    fmt::fmt
+    websockets
+)