feat: add mesh network visualizer

- D3.js force-directed graph for real-time mesh visualization
- WebSocket server (mesh-viz-bridge crate) for live updates
- Demo mode with simulated topology
- JSONL file upload for offline analysis
- Optional viz logging in mesh_node forwarding

Cargo.toml

@@ -14,6 +14,8 @@ members = [
     "crates/quicprochat-p2p",
     # Generic decentralized service layer (FAPP, Housing, etc.)
     "crates/meshservice",
+    # WebSocket bridge for viz/mesh-graph.html (tails NDJSON → browsers)
+    "viz/bridge",
 ]
 
 [workspace.package]