fix(runtime): a root actor panic escapes run()

The trampoline caught the root's panic, recorded it as Outcome::Panic on
the slot, and run() dropped the initial handle without reading it — every
assert inside run(), the standard test-suite pattern, was silently
vacuous (found live: a failing-first test passed). run() now reads the
root outcome before the handle drop and resume_unwinds the payload after
full teardown, so a caller's catch_unwind leaves the Runtime reusable;
Exit and Stopped return normally. The payload message is printed before
re-raising (the throw-site hook output was suppressed in-actor).

Correct the two tests this unmasked, both born failing and never run:
the select loser-arm test kept a closed arm in the set (the documented
closed-arm rule: a closed arm reports ready forever — observe the
disconnect and drop it); the send_after-to-dead test expected Ok(None)
from a closed+empty channel (documented: Err(RecvError), which proves
nothing-delivered even more strongly).

tests/timer.rs

@@ -401,8 +401,10 @@ fn send_after_to_dead_typed_pid_is_silent() {
         assert_eq!(report_rx.recv().unwrap(), 1); // sink has now exited
         let _id = send_after(Duration::from_millis(15), sink, 2);
         sleep(Duration::from_millis(45)); // let it fire against the dead pid
-        // No panic, and nothing further delivered.
-        assert_eq!(report_rx.try_recv(), Ok(None));
+        // No panic; the sink is gone, so its report sender dropped with it —
+        // closed+empty is Err (documented), which also proves nothing
+        // further was delivered.
+        assert!(report_rx.try_recv().is_err(), "nothing further delivered");
     });
 }