gen_statem: GenStatem* type prefix + cleanup

- rename StatemRef/StatemCallError/StatemSendError -> GenStatem*
- move the inline unit test out of src; consolidate the Switch coverage
  onto a single macro-driven harness in tests/gen_statem.rs
- drop the redundant hand-written Switch test machine and the two
  untracked rejected-direction probes (succ_enums, typed_edges)
- rename examples statem_{fused,macro}.rs -> gen_statem_{fused,macro}.rs
- strip RFC/chunk/spike provenance and fix the mislabeled "throwaway"
  example header and dead cross-references

src/lib.rs

@@ -59,8 +59,8 @@ pub use gen_server::{
     NamedGenServerBuilder, GenServerBuilder, GenServerCtx, GenServerName, GenServerRef, TimerHandle, Watcher,
 };
 pub use gen_statem::{
-    CallError as StatemCallError, Cx, Machine, Reply, Resolution, SendError as StatemSendError,
-    StatemRef,
+    CallError as GenStatemCallError, Cx, Machine, Reply, Resolution, SendError as GenStatemSendError,
+    GenStatemRef,
 };
 pub use introspect::{
     actor_info, snapshot, tree, tree_from, ActorInfo, ActorState, RuntimeSnapshot, RuntimeTree,