Mirrors the gen_server surface in gen_statem's event model:
- Cx::trap_exit() (in the initial enter): a shutdown request then arrives
as the Shutdown event, routed by state through `shutdown` rows (default
for a state with no row: stop); linked-peer deaths as `exit <pat>` rows
(default: drop). Non-trapping machines are stopped outright, as before.
- Cx::stop(): normal self-exit after the current event; `stop` tail keyword
is sugar for { cx.stop(); prev }.
- Machine::terminate (optional `terminate { … }` macro block), run from a
Drop guard on every exit path; the guard also drains armed timers.
- Machine::shutdown_ev / exit_ev (defaults None) so hand-written machines
keep compiling; GenStatemRef::shutdown() is graceful and waits.
- Loop selects exits > timers > inbox.
Tests: tests/gen_statem_shutdown.rs.
