feat(p2p): add MeshNode integrating all production modules

New mesh_node.rs providing a production-ready node:
- MeshNodeBuilder for fluent configuration
- MeshConfig integration for all settings
- MeshMetrics tracking for all operations
- Rate limiting on incoming messages
- Backpressure controller
- Graceful shutdown via ShutdownCoordinator
- Optional FappRouter based on capabilities
- MeshRouter for envelope routing
- TransportManager for multi-transport support

Key APIs:
- MeshNodeBuilder::new().fapp_relay().build()
- node.process_incoming() with rate limiting + metrics
- node.gc() for store/routing table cleanup
- node.shutdown() for graceful termination

222 tests passing (203 lib + 3 fapp_flow + 16 multi_node)

Cargo.toml

@@ -12,6 +12,8 @@ members = [
     # P2P crate uses iroh (~90 extra deps). Only compiled when the `mesh`
     # feature is enabled on quicprochat-client.
     "crates/quicprochat-p2p",
+    # Generic decentralized service layer (FAPP, Housing, etc.)
+    "crates/meshservice",
 ]
 
 [workspace.package]