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)

.gitignore

@@ -1 +1,13 @@
 ref_arch.pdf
+build/
+.cache/
+CMakeUserPresets.json
+compile_commands.json
+.vcpkg/
+vcpkg_installed/
+node_modules/
+web/dist/
+*.o
+*.a
+*.so
+*.d