feat(mesh): add MeshEnvelopeV2 with truncated 16-byte addresses

S5: Compact envelope format for constrained links:
- 16-byte truncated addresses (MeshAddress) instead of 32-byte keys
- 16-byte truncated content ID
- u16 TTL and u32 timestamp (smaller than V1)
- Priority field (Low/Normal/High/Emergency)
- ~30-50 bytes savings per envelope vs V1

Full public keys are exchanged during announce phase and cached in
routing table. Envelope only needs addresses for routing.

crates/quicprochat-p2p/src/lib.rs

@@ -17,6 +17,7 @@ pub mod announce;
 pub mod announce_protocol;
 pub mod broadcast;
 pub mod envelope;
+pub mod envelope_v2;
 pub mod identity;
 pub mod link;
 pub mod mesh_router;