feat: load pipeline graph from graph.json

studio-manager now accepts an optional path argument:
  dmf-studio-manager graph.json

load_graph() parses nodes and edges from JSON, generating fresh UUIDs
for each MXL flow on every launch. Falls back to hardcoded build_graph()
when no argument is given.

graph.json at repo root defines the current testpattern→ndiout pipeline.

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

graph.json

@@ -0,0 +1,18 @@
+{
+  "nodes": [
+    { "id": "testpattern", "type": "testpattern", "params": { "pattern": "bars" } },
+    { "id": "ndiout",      "type": "ndiout",      "params": {} }
+  ],
+  "edges": [
+    {
+      "from": "testpattern", "from_port": "flow_id",
+      "to":   "ndiout",      "to_port":   "flow_id",
+      "format": { "kind": "video", "width": 1920, "height": 1080, "fps_num": 25, "fps_den": 1 }
+    },
+    {
+      "from": "testpattern", "from_port": "audio_flow_id",
+      "to":   "ndiout",      "to_port":   "audio_flow_id",
+      "format": { "kind": "audio", "sample_rate": 48000, "channels": 2, "bit_depth": 32 }
+    }
+  ]
+}