pamoja-session 0.1.11

Encrypted, authenticated sessions for pamoja: X25519 key agreement (RFC 7748) and HKDF-SHA256 (RFC 5869) derive a session key, then ChaCha20-Poly1305 (RFC 8439) protects each message with counter nonces and an anti-replay window, so two devices that know each other's keys share a confidential, tamper-evident link, no_std and allocation-free. The secured-channel half ahead of the rustls/DTLS driver.