feat(mesh): add KeyPackage distribution over mesh

Implements announce-based KeyPackage distribution for serverless MLS:

- MeshAnnounce now includes optional `keypackage_hash` field (8 bytes)
- CAP_MLS_READY capability flag for nodes with KeyPackages
- KeyPackageCache for storing received KeyPackages:
  - Indexed by mesh address
  - Multiple per address (for rotation)
  - TTL-based expiry
  - Capacity-bounded with LRU eviction
- Mesh protocol messages:
  - KeyPackageRequest (request by address or hash)
  - KeyPackageResponse (KeyPackage + hash)
  - KeyPackageUnavailable (negative response)

Protocol flow:
1. Bob announces with keypackage_hash
2. Alice requests KeyPackage via mesh
3. Bob (or relay) responds with full KeyPackage
4. Alice creates MLS Welcome, sends to Bob via mesh

crates/quicprochat-p2p/src/lib.rs

@@ -20,6 +20,8 @@ pub mod fapp_router;
 pub mod broadcast;
 pub mod envelope;
 pub mod envelope_v2;
+pub mod keypackage_cache;
+pub mod mesh_protocol;
 pub mod mls_lite;
 pub mod identity;
 pub mod link;