feat(p2p): add production infrastructure modules

- error.rs: Structured error types with context for all subsystems
  (transport, routing, crypto, protocol, store, config)
- config.rs: Runtime configuration with TOML parsing and validation
- metrics.rs: Counter/gauge/histogram metrics with transport-specific
  tracking and JSON-serializable snapshots
- rate_limit.rs: Token bucket rate limiting with per-peer tracking,
  duty cycle enforcement for LoRa, and backpressure control

These modules provide the foundation for production deployment.

crates/quicprochat-p2p/src/lib.rs

@@ -15,7 +15,9 @@
 pub mod address;
 pub mod announce;
 pub mod announce_protocol;
+pub mod config;
 pub mod crypto_negotiation;
+pub mod error;
 pub mod fapp;
 pub mod fapp_router;
 pub mod broadcast;
@@ -23,7 +25,9 @@ pub mod envelope;
 pub mod envelope_v2;
 pub mod keypackage_cache;
 pub mod mesh_protocol;
+pub mod metrics;
 pub mod mls_lite;
+pub mod rate_limit;
 pub mod identity;
 pub mod link;
 pub mod mesh_router;