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)

vcpkg.json

@@ -0,0 +1,20 @@
+{
+  "name": "dmf-studio",
+  "version": "0.1.0",
+  "dependencies": [
+    "libwebsockets",
+    "nlohmann-json",
+    "spdlog",
+    "fmt",
+    "catch2",
+    {
+      "name": "stduuid",
+      "version>=": "1.2.3",
+      "features": ["system-gen", "gsl-span"]
+    },
+    "picojson",
+    "cli11",
+    "ada-url"
+  ],
+  "builtin-baseline": "432412eecac55981cf608cc12e5b4bd91768ec57"
+}