feat(p2p): add persistence and graceful shutdown

- persistence.rs: Append-only log storage for routing table,
  KeyPackage cache, and messages with compaction and GC
- shutdown.rs: Coordinated shutdown with phase transitions,
  task tracking, connection draining, and hook system

Enables stateful operation and clean restarts.

crates/quicprochat-p2p/src/lib.rs

@@ -27,7 +27,9 @@ pub mod keypackage_cache;
 pub mod mesh_protocol;
 pub mod metrics;
 pub mod mls_lite;
+pub mod persistence;
 pub mod rate_limit;
+pub mod shutdown;
 pub mod identity;
 pub mod link;
 pub mod mesh_router;