//! Post-quantum link mode (`ARCHITECTURE.md` §6.2). Feature-gated: `pq-links`.
//!
//! After the standard 3-packet Reticulum handshake, extension-aware peers
//! upgrade the link: the initiator sends an ML-KEM-768 encapsulation
//! (context `EXT_LINK_UPGRADE`); the responder decapsulates and replies with an
//! ML-DSA-65-signed confirmation. The link `Token` is then rekeyed via HKDF over
//! `classical_shared || pq_shared` — defense in depth: an attacker must break
//! both ECDH and ML-KEM.
//!
//! Backend crate selection (`pqcrypto-mlkem`/`pqcrypto-mldsa` vs RustCrypto's
//! `ml-kem`/`ml-dsa`) is deferred to Phase 4 (open question §12.5). Implemented
//! in Phase 4 (§11.4).
