saveSessionPasskey

Persist the latched session passkey for this transport identity.

The engine no longer calls this (since 0.2.0): session passkeys are per-node and in-memory only — the local node's passkey is never required (firmware rewrites phone packets to from = 0, which is passkey-exempt), and remote-node passkeys expire after ~4 minutes, making persistence useless across process restarts. Retained as a host-facing capability. Implementations must overwrite any previous passkey atomically.