feat: add node HTTP control, engine-to-node communication, connect-input/output API

- Node control server now accepts HTTP POST /cmd for command dispatch
  (in addition to existing WebSocket control)
- Added NodeControlClient: engine sends commands to nodes via HTTP POST
- New REST endpoints:
  POST /api/graph/nodes/:id/connect-input  - connect node input to MXL flow
  POST /api/graph/nodes/:id/connect-output - create new MXL flow + connect output
  POST /api/graph/nodes/:id/disconnect-port - remove reader/writer
  POST /api/graph/nodes/:id/command - send raw command to node
- Flow IDs now use proper UUID v4 format (MXL requires standard UUIDs)
- Flow definitions use NMOS format (urn:x-nmos:format:video)
- Engine passes --mxl-domain to node processes
- User-provided node IDs (via 'id' field in POST body)
- End-to-end verified: testsrc → passthrough → new MXL output flow

libs/dmf-engine/include/dmf-engine/node_control_client.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <string>
+#include <unordered_map>
+#include <cstdint>
+
+namespace dmf_engine {
+
+class NodeControlClient {
+public:
+    bool send_command(uint16_t port, const std::string& json_cmd);
+
+    void register_node(const std::string& node_id, uint16_t port);
+    void unregister_node(const std::string& node_id);
+    uint16_t get_port(const std::string& node_id) const;
+
+private:
+    std::unordered_map<std::string, uint16_t> node_ports_;
+};
+
+} // namespace dmf_engine