feat(mesh): add MLS-Lite to full MLS upgrade path

crypto_negotiation module enables transitioning between crypto modes:

GroupCryptoState tracks current mode:
- MlsLite (signed/unsigned)
- FullMls (classical/hybrid)
- Upgrading (transition state)

MlsLiteBootstrap derives MLS-Lite keys from MLS epoch secret:
- Enables fallback to MLS-Lite over constrained links
- Same group can use full MLS over WiFi, MLS-Lite over LoRa

Upgrade protocol:
1. Member sends KeyPackage over fast link
2. Creator creates MLS Welcome
3. Group transitions to full MLS
4. Optionally maintains MLS-Lite fallback for constrained links

crates/quicprochat-p2p/src/lib.rs

@@ -15,6 +15,7 @@
 pub mod address;
 pub mod announce;
 pub mod announce_protocol;
+pub mod crypto_negotiation;
 pub mod fapp;
 pub mod fapp_router;
 pub mod broadcast;