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/runtime.rs

@@ -485,3 +485,35 @@ fn multi_thread_timer_only_no_pipe_contention() {
         SLEEP_MS * 2,
     );
 }
+
+// ---------------------------------------------------------------------------
+// Root panic propagation
+
+/// A panic in the root actor escapes `run()` to the caller. Anything else
+/// makes every assert inside `run` silently vacuous — found live when a
+/// failing-first test passed: the tripped assert was caught by the
+/// trampoline, recorded as `Outcome::Panic` on the root slot, and dropped
+/// unread with the initial handle.
+#[test]
+#[should_panic(expected = "root actor panic escapes")]
+fn root_panic_escapes_run() {
+    rt1().run(|| {
+        panic!("root actor panic escapes");
+    });
+}
+
+/// Teardown completes before the root panic propagates: a caller that
+/// catches it can immediately `run()` again on the same `Runtime` (the
+/// documented sequential-reuse contract).
+#[test]
+fn runtime_reusable_after_root_panic() {
+    let r = rt1();
+    let caught = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+        r.run(|| panic!("boom"));
+    }));
+    assert!(caught.is_err(), "root panic must escape run()");
+    let ran = Arc::new(AtomicBool::new(false));
+    let ran_t = ran.clone();
+    r.run(move || ran_t.store(true, Ordering::Relaxed));
+    assert!(ran.load(Ordering::Relaxed), "runtime unusable after root panic");
+}