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)

engine/src/main.cpp

@@ -0,0 +1,46 @@
+#include <dmf-engine/api_server.hpp>
+#include <dmf-engine/graph.hpp>
+#include <dmf-engine/flow_manager.hpp>
+#include <dmf-engine/process_manager.hpp>
+
+#include <spdlog/spdlog.h>
+
+#include <csignal>
+#include <atomic>
+
+static std::atomic<bool> g_running{true};
+
+static void signal_handler(int /*signum*/) {
+    g_running = false;
+}
+
+int main(int argc, char* argv[]) {
+    uint16_t port = 8080;
+
+    for (int i = 1; i < argc; ++i) {
+        std::string arg = argv[i];
+        if ((arg == "--port" || arg == "-p") && i + 1 < argc) {
+            port = static_cast<uint16_t>(std::stoi(argv[++i]));
+        }
+    }
+
+    std::signal(SIGINT, signal_handler);
+    std::signal(SIGTERM, signal_handler);
+
+    spdlog::info("DMF Studio Engine starting on port {}", port);
+
+    dmf_engine::Graph graph;
+    dmf_engine::FlowManager flow_manager;
+    dmf_engine::ProcessManager process_manager;
+
+    dmf_engine::ApiServer api_server(port, graph, flow_manager, process_manager);
+
+    spdlog::info("DMF Studio Engine ready");
+
+    while (g_running) {
+        api_server.poll(100);
+    }
+
+    spdlog::info("DMF Studio Engine shutting down");
+    return 0;
+}