feat: graph-level fanout — reuse flow UUID for same from_node+from_port

Edges that share the same source node and port now get the same MXL flow
UUID. One writer, multiple independent readers — each tracks its own
position in the ring buffer. Enables 1-to-N routing in graph.json without
any splitter node.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

studio-manager/main.cpp

@@ -6,8 +6,10 @@
 #include <cstdlib>
 #include <filesystem>
 #include <fstream>
+#include <map>
 #include <sstream>
 #include <string>
+#include <utility>
 #include <vector>
 #include <fcntl.h>
 #include <sys/wait.h>
@@ -62,11 +64,18 @@ static dmf::FlowGraph load_graph(const std::string& path) {
             n.value("params", nlohmann::json::object())
         });
     }
+    // Edges sharing the same from_node+from_port reuse the same flow UUID,
+    // so one writer can be read by multiple consumers (MXL supports N readers per flow).
+    std::map<std::pair<std::string,std::string>, std::string> flow_ids;
     for (const auto& e : j.at("edges")) {
+        auto from_node = e.at("from").get<std::string>();
+        auto from_port = e.at("from_port").get<std::string>();
+        auto key = std::make_pair(from_node, from_port);
+        if (!flow_ids.count(key)) flow_ids[key] = gen_uuid();
         g.edges.push_back({
-            gen_uuid(),
-            e.at("from").get<std::string>(),
-            e.at("from_port").get<std::string>(),
+            flow_ids.at(key),
+            from_node,
+            from_port,
             e.value("to",      std::string{}),
             e.value("to_port", std::string{}),
             e.at("format")