gen_statem: state and named timeouts, info events

Add the two timeout flavours, both surfacing as ordinary events matched
in on-state arms:

- cx.state_timeout(d): fires a state_timeout event after d in the current
  state, auto-reset by the loop on every real transition.
- cx.timeout(name, d): fires a timeout(name) event after d, surviving state
  changes, keyed by name, with cx.cancel_timeout(name).

Both ride the existing timer min-heap via send_after_to onto a new per-loop
system channel, selected above the inbox so a fire can't be starved by inbox
traffic. A local-id stamp on each fire lets a reset/cancel that loses the race
discard a stale fire. The macro grows an info: clause and folds three internal
Ev variants (Info, StateTimeout, Timeout) alongside cast/call, with new row
keywords info / state_timeout / timeout. Unmatched info silently drops (the
gen_server default); state/named timeouts have no default, so a state that can
see one must handle it or the match is non-exhaustive.

Rename the hand-written expansion-target example fused -> expanded, retire the
deprecated Switch demo machine (its round-trip / enter / panic-down coverage
moves onto the timer machine), and refresh the macro docs to the door machine.

cargo build --all-targets warning-free; cargo test green.

examples/gen_statem_macro.rs

@@ -1,4 +1,4 @@
-//! The **same** machine as `examples/gen_statem_fused.rs`, written through the
+//! The **same** machine as `examples/gen_statem_expanded.rs`, written through the
 //! `gen_statem!` macro. Diff this file against that one to see exactly what the
 //! macro buys: every `// ===` section there that was boilerplate (the `Ev`
 //! enum, the `DoorSm` struct, `start`, the whole `Machine` impl, the `enter`
@@ -21,7 +21,7 @@ use smarm::gen_statem;
 use smarm::run;
 use smarm::gen_statem::Reply;
 
-// === user types (identical to gen_statem_fused.rs) =========================
+// === user types (identical to gen_statem_expanded.rs) =========================
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 enum Door {
@@ -87,7 +87,7 @@ fn on_unlock(key: u32) -> UnlockOutcome {
 
 gen_statem! {
     machine: DoorSm { state: Door, data: Data };
-    event:   Ev { cast: Cast, call: Call };
+    event:   Ev { cast: Cast, call: Call, info: () };
 
     // You name the bindings the bodies use; the macro can't lend you its own
     // `self`/`cx` across macro hygiene. `data` = &mut Data, `prev` = current
@@ -117,6 +117,10 @@ gen_statem! {
         call Call::GetState(r)  => { r.reply(prev); prev },
         call Call::GetEnters(r) => { r.reply(data.enters); prev },
         call Call::GetPushes(r) => { r.reply(data.pushes); prev },
+        // This machine arms no timeouts, so refuse them everywhere. (Info has a
+        // built-in silent-drop default, so it needs no row.)
+        state_timeout          => unhandled,
+        timeout _              => unhandled,
     }
 }
 
@@ -145,7 +149,7 @@ fn main() {
 
 // ===========================================================================
 // BREAK-CASE MENU — the four guarantees, through the macro. Each fires exactly
-// as it does in the hand-written gen_statem_fused.rs.
+// as it does in the hand-written gen_statem_expanded.rs.
 //
 // 1. ORPHAN HANDLER (dead_code -> error):
 //      add `fn on_slam(_d: &mut Data) {}` and don't reference it.