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/Cargo.toml

@@ -37,6 +37,10 @@ x25519-dalek     = { workspace = true }
 hkdf             = { workspace = true }
 thiserror        = { workspace = true }
 
+# Configuration
+toml             = "0.8"
+humantime-serde  = "1"
+
 [dev-dependencies]
 tempfile = "3"