feat(mesh): add MLS-Lite lightweight encryption for constrained links

MLS-Lite provides group encryption without full MLS overhead:
- Pre-shared group secret (QR code, NFC, or MLS epoch export)
- ChaCha20-Poly1305 symmetric encryption (same as MLS app messages)
- Per-message nonce from epoch + sequence
- Replay protection via sliding window
- Optional Ed25519 signatures

Wire overhead: ~41 bytes without signature, ~105 with signature
(vs ~174 bytes for MeshEnvelope V1)

Tradeoffs vs full MLS:
- No automatic post-compromise security (manual key rotation)
- No automatic forward secrecy (only per-epoch)
- Keys are pre-shared, not negotiated

Designed for SF12 LoRa where MLS KeyPackages are impractical.

crates/quicprochat-p2p/src/lib.rs

@@ -18,6 +18,7 @@ pub mod announce_protocol;
 pub mod broadcast;
 pub mod envelope;
 pub mod envelope_v2;
+pub mod mls_lite;
 pub mod identity;
 pub mod link;
 pub mod mesh_router;