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.lock

@@ -3202,6 +3202,21 @@ version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8ca58f447f06ed17d5fc4043ce1b10dd205e060fb3ce5b979b8ed8e59ff3f79"
 
+[[package]]
+name = "meshservice"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "ciborium",
+ "ed25519-dalek 2.2.0",
+ "rand 0.8.5",
+ "serde",
+ "sha2 0.10.9",
+ "thiserror 1.0.69",
+ "tokio",
+ "x25519-dalek",
+]
+
 [[package]]
 name = "metrics"
 version = "0.22.4"