RFC 017 chunk 1: gen_statem primitives (no macro yet)

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.

src/lib.rs

@@ -27,6 +27,7 @@ pub mod registry;
 pub mod pg;
 pub mod link;
 pub mod gen_server;
+pub mod statem;
 pub mod introspect;
 #[cfg(feature = "observer")]
 pub mod observer;
@@ -57,6 +58,10 @@ pub use gen_server::{
     call, cast, shutdown, whereis_server, CallError, CallTimeoutError, CastError, GenServer,
     NamedServerBuilder, ServerBuilder, ServerCtx, ServerName, ServerRef, TimerHandle, Watcher,
 };
+pub use statem::{
+    CallError as StatemCallError, Cx, Machine, Reply, Resolution, SendError as StatemSendError,
+    StatemRef,
+};
 pub use introspect::{
     actor_info, snapshot, tree, tree_from, ActorInfo, ActorState, RuntimeSnapshot, RuntimeTree,
     TreeNode, SNAPSHOT_FORMAT_VERSION,