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/src/graph.cpp

@@ -0,0 +1,135 @@
+#include <dmf-engine/graph.hpp>
+
+#include <nlohmann/json.hpp>
+
+#include <spdlog/spdlog.h>
+
+namespace dmf_engine {
+
+NodeId Graph::add_node(const std::string& type, const nlohmann::json& config) {
+    auto id = type + "_" + std::to_string(next_node_num_++);
+    GraphNode node;
+    node.id = id;
+    node.type = type;
+    node.config = config;
+    node.state = NodeState::Stopped;
+    nodes_[id] = std::move(node);
+    spdlog::info("Graph: added node '{}' type='{}'", id, type);
+    return id;
+}
+
+bool Graph::remove_node(const NodeId& node_id) {
+    auto it = nodes_.find(node_id);
+    if (it == nodes_.end()) {
+        spdlog::warn("Graph: node '{}' not found", node_id);
+        return false;
+    }
+
+    std::vector<EdgeId> edges_to_remove;
+    for (const auto& [eid, edge] : edges_) {
+        if (edge.from_node == node_id || edge.to_node == node_id) {
+            edges_to_remove.push_back(eid);
+        }
+    }
+    for (const auto& eid : edges_to_remove) {
+        edges_.erase(eid);
+        spdlog::info("Graph: removed edge '{}' (connected to removed node '{}')", eid, node_id);
+    }
+
+    nodes_.erase(it);
+    spdlog::info("Graph: removed node '{}'", node_id);
+    return true;
+}
+
+const GraphNode* Graph::get_node(const NodeId& node_id) const {
+    auto it = nodes_.find(node_id);
+    return it != nodes_.end() ? &it->second : nullptr;
+}
+
+std::vector<GraphNode> Graph::get_nodes() const {
+    std::vector<GraphNode> result;
+    for (const auto& [_, node] : nodes_) {
+        result.push_back(node);
+    }
+    return result;
+}
+
+EdgeId Graph::add_edge(const NodeId& from_node, const PortId& from_port,
+                       const NodeId& to_node, const PortId& to_port,
+                       const FlowId& flow_id, const nlohmann::json& flow_def) {
+    auto id = from_node + ":" + from_port + "->" + to_node + ":" + to_port;
+    GraphEdge edge;
+    edge.id = id;
+    edge.from_node = from_node;
+    edge.from_port = from_port;
+    edge.to_node = to_node;
+    edge.to_port = to_port;
+    edge.flow_id = flow_id;
+    edge.flow_def = flow_def;
+    edges_[id] = std::move(edge);
+    spdlog::info("Graph: added edge '{}' flow_id={}", id, flow_id);
+    return id;
+}
+
+bool Graph::remove_edge(const EdgeId& edge_id) {
+    auto it = edges_.find(edge_id);
+    if (it == edges_.end()) {
+        spdlog::warn("Graph: edge '{}' not found", edge_id);
+        return false;
+    }
+    edges_.erase(it);
+    spdlog::info("Graph: removed edge '{}'", edge_id);
+    return true;
+}
+
+std::vector<GraphEdge> Graph::get_edges() const {
+    std::vector<GraphEdge> result;
+    for (const auto& [_, edge] : edges_) {
+        result.push_back(edge);
+    }
+    return result;
+}
+
+std::vector<GraphEdge> Graph::get_edges_for_node(const NodeId& node_id) const {
+    std::vector<GraphEdge> result;
+    for (const auto& [_, edge] : edges_) {
+        if (edge.from_node == node_id || edge.to_node == node_id) {
+            result.push_back(edge);
+        }
+    }
+    return result;
+}
+
+nlohmann::json Graph::serialize() const {
+    auto j = nlohmann::json::object();
+
+    auto nodes_arr = nlohmann::json::array();
+    for (const auto& [id, node] : nodes_) {
+        nodes_arr.push_back({
+            {"id", node.id},
+            {"type", node.type},
+            {"config", node.config},
+            {"state", static_cast<int>(node.state)},
+            {"control_port", node.control_port},
+            {"pid", node.pid},
+        });
+    }
+    j["nodes"] = nodes_arr;
+
+    auto edges_arr = nlohmann::json::array();
+    for (const auto& [id, edge] : edges_) {
+        edges_arr.push_back({
+            {"id", edge.id},
+            {"from_node", edge.from_node},
+            {"from_port", edge.from_port},
+            {"to_node", edge.to_node},
+            {"to_port", edge.to_port},
+            {"flow_id", edge.flow_id},
+        });
+    }
+    j["edges"] = edges_arr;
+
+    return j;
+}
+
+} // namespace dmf_engine