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)

tests/test_graph.cpp

@@ -0,0 +1,88 @@
+#include <dmf-engine/graph.hpp>
+#include <dmf-engine/flow_manager.hpp>
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("Graph add and remove nodes", "[graph]") {
+    dmf_engine::Graph graph;
+
+    auto id1 = graph.add_node("passthrough");
+    auto id2 = graph.add_node("test-source");
+
+    REQUIRE(graph.get_node(id1) != nullptr);
+    REQUIRE(graph.get_node(id2) != nullptr);
+    REQUIRE(graph.get_node(id1)->type == "passthrough");
+    REQUIRE(graph.get_node(id2)->type == "test-source");
+
+    auto nodes = graph.get_nodes();
+    REQUIRE(nodes.size() == 2);
+
+    graph.remove_node(id1);
+    REQUIRE(graph.get_node(id1) == nullptr);
+    REQUIRE(graph.get_nodes().size() == 1);
+}
+
+TEST_CASE("Graph add and remove edges", "[graph]") {
+    dmf_engine::Graph graph;
+
+    auto n1 = graph.add_node("test-source");
+    auto n2 = graph.add_node("passthrough");
+
+    auto edge_id = graph.add_edge(n1, "video_out", n2, "video_in", "flow1", {});
+    REQUIRE(graph.get_edges().size() == 1);
+
+    graph.remove_edge(edge_id);
+    REQUIRE(graph.get_edges().size() == 0);
+}
+
+TEST_CASE("Graph remove node also removes edges", "[graph]") {
+    dmf_engine::Graph graph;
+
+    auto n1 = graph.add_node("test-source");
+    auto n2 = graph.add_node("passthrough");
+
+    graph.add_edge(n1, "video_out", n2, "video_in", "flow1", {});
+    graph.add_edge(n2, "video_out", n1, "video_in", "flow2", {});
+
+    REQUIRE(graph.get_edges().size() == 2);
+
+    graph.remove_node(n1);
+    REQUIRE(graph.get_edges().size() == 0);
+}
+
+TEST_CASE("FlowManager creates unique IDs", "[flow_manager]") {
+    dmf_engine::FlowManager fm1;
+    dmf_engine::FlowManager fm2;
+
+    auto id1 = fm1.create_flow_id();
+    auto id2 = fm1.create_flow_id();
+    auto id3 = fm2.create_flow_id();
+
+    REQUIRE(id1 != id2);
+    REQUIRE(id2 != id3);
+}
+
+TEST_CASE("FlowManager creates V210 flow definition", "[flow_manager]") {
+    dmf_engine::FlowManager fm;
+    auto flow_id = fm.create_flow_id();
+    auto def = fm.create_v210_flow_def(flow_id, 1920, 1080, 50, 1);
+
+    REQUIRE(def["id"] == flow_id);
+    REQUIRE(def["format"] == "video/v210");
+    REQUIRE(def["width"] == 1920);
+    REQUIRE(def["height"] == 1080);
+}
+
+TEST_CASE("Graph serialize", "[graph]") {
+    dmf_engine::Graph graph;
+
+    auto n1 = graph.add_node("test-source");
+    auto n2 = graph.add_node("passthrough");
+    graph.add_edge(n1, "video_out", n2, "video_in", "flow1", {});
+
+    auto j = graph.serialize();
+    REQUIRE(j.contains("nodes"));
+    REQUIRE(j.contains("edges"));
+    REQUIRE(j["nodes"].size() == 2);
+    REQUIRE(j["edges"].size() == 1);
+}