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.
