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

engine/src/main.cpp

@@ -2,6 +2,7 @@
 #include <dmf-engine/graph.hpp>
 #include <dmf-engine/flow_manager.hpp>
 #include <dmf-engine/process_manager.hpp>
+#include <dmf-engine/node_control_client.hpp>
 
 #include <spdlog/spdlog.h>
 
@@ -16,11 +17,19 @@ static void signal_handler(int /*signum*/) {
 
 int main(int argc, char* argv[]) {
     uint16_t port = 8080;
+    std::string mxl_domain = "/dev/shm/mxl";
 
     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]));
+        } else if ((arg == "--mxl-domain" || arg == "-d") && i + 1 < argc) {
+            mxl_domain = argv[++i];
+        } else if (arg == "--help" || arg == "-h") {
+            spdlog::info("Usage: dmf-studio-engine [options]");
+            spdlog::info("  --port, -p          API server port (default: 8080)");
+            spdlog::info("  --mxl-domain, -d    MXL domain path (default: /dev/shm/mxl)");
+            return 0;
         }
     }
 
@@ -32,8 +41,9 @@ int main(int argc, char* argv[]) {
     dmf_engine::Graph graph;
     dmf_engine::FlowManager flow_manager;
     dmf_engine::ProcessManager process_manager;
+    dmf_engine::NodeControlClient control_client;
 
-    dmf_engine::ApiServer api_server(port, graph, flow_manager, process_manager);
+    dmf_engine::ApiServer api_server(port, graph, flow_manager, process_manager, control_client, mxl_domain);
 
     spdlog::info("DMF Studio Engine ready");