Runtime support layer for gen_statem, built against existing public API
(channel + scheduler::spawn), sibling to gen_server. No macro: per review,
build the primitives first and hand-write the Switch example to evaluate
whether a statem! macro earns its place before committing to one.
- src/statem.rs: Machine trait (on_start/handle), Resolution<S> with
From<S>, Cx (on_unhandled), Reply<T> move-only reply handle, StatemRef
(send/call), spawn + inbox loop. Real time only; Postpone + Cx timeout
arming are in the type surface but not yet acted on (chunks 2-3).
- examples/statem_switch.rs: the RFC Switch machine hand-written against the
primitives, tagged USER vs MACRO to mark what a macro would generate.
Asserts the RFC end-state (flips=1, enters=3).
- tests/statem.rs: call/cast round-trip, enter-on-start/transition-not-stay,
panicking-handler -> Down.
Reply<T> included (the call helper needs a handle type; keeps the example
true to the RFC surface) but isolated and trivially removable if we drop it.
