fix: emit watch notification when StopSignal stops an actor

StopSignal interception in tick_all set stopping=true but did not push
to the deaths vector, so phase 5b watch notifications never fired for
externally-stopped actors (via rt.stop_actor()). Now pushes
(addr, ExitReason::Stopped) to deaths, consistent with the
ctx.stop_self() path.

Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski

crates/wasm-actor/tests/wasm_actor.rs

@@ -610,7 +610,6 @@ impl ActorInterface for ExitWatcher {
 }
 
 #[test]
-#[ignore] // BUG: StopSignal handling in tick_all doesn't push to deaths vec — watchers never notified
 fn native_watcher_notified_when_wasm_actor_stops() {
     // A native actor watches a WASM actor. When the WASM actor is stopped,
     // the watcher should receive ActorExited with ExitReason::Stopped.