Both ran scram verify_plain (~1s PBKDF2 at 1.2M iters) inline under the engine
lock, freezing the whole daemon per login. Add NetAction::DeferAuthenticate +
AuthThen continuation + ctx.defer_authenticate, mirroring DeferPassword: the
module/SASL path fetches the verifier cheaply and defers; the link layer runs
verify_plain on spawn_blocking, then Engine::complete_authenticate finishes the
login (IDENTIFY reuses the same ctx helpers, so its login/AJOIN/vhost/memo
side-effects are unchanged). Tests resolve the defer inline (cfg(test)). The
gRPC web-login path was already fixed; no login path stalls services now.
(GHOST/DROP/CERT/GROUP still verify inline but are rare account ops, not logins.)
engine/mod.rs had grown past 5k lines, most of it one test module and
a few large method clusters. Lift the tests into engine/tests.rs, and
move the cohesive method groups into sibling files — sasl, kicker
(chat policing), register (account-authority relay), and dispatch
(service routing and fantasy). The Engine type, its master event
handler, shared helpers, and the small state structs stay in mod.rs;
each sibling adds its own impl Engine block. Pure reorganisation: no
behaviour change, tests unchanged and green.
