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)

crates/quicprochat-p2p/src/lib.rs

@@ -32,6 +32,7 @@ pub mod rate_limit;
 pub mod shutdown;
 pub mod identity;
 pub mod link;
+pub mod mesh_node;
 pub mod mesh_router;
 pub mod routing;
 pub mod routing_table;