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).

src/runtime.rs

@@ -1013,6 +1013,11 @@ pub fn init(config: Config) -> Runtime {
 impl Runtime {
     /// Run `f` as the initial actor, block until all actors finish.
     /// Can be called multiple times sequentially on the same `Runtime`.
+    ///
+    /// A panic in the initial actor propagates out of `run` (re-raised
+    /// after teardown, so the `Runtime` stays reusable even under a
+    /// caller's `catch_unwind`). Panics in other actors surface through
+    /// joins, links, monitors, and supervisors as usual.
     pub fn run(&self, f: impl FnOnce() + Send + 'static) {
         // Install smarm's panic hook on first call. The default Rust hook is
         // not reentrant — concurrent actor panics can trigger a double-panic
@@ -1114,6 +1119,23 @@ impl Runtime {
             let _ = t.join();
         }
 
+        // The root's outcome, read before its handle drops (the outstanding
+        // handle pins the slot: no reclaim, no generation change under us).
+        // A root panic must escape `run()` — propagated at the tail below,
+        // after teardown. Dropping it unread here made every assert inside
+        // `run` silently vacuous (found live: a failing-first test passed).
+        let root_outcome = {
+            let slot = match self.inner.slot_at(initial_handle.pid()) {
+                Some(slot) => slot,
+                None => panic!("run(): root pid out of range"),
+            };
+            debug_assert!(
+                slot.status_for(initial_handle.pid()) == Status::Done,
+                "root not Done at run() teardown"
+            );
+            slot.cold.lock().outcome.take()
+        };
+
         // Drop initial handle (decrements outstanding_handles count).
         drop(initial_handle);
 
@@ -1147,6 +1169,26 @@ impl Runtime {
         // Flush trace to disk (no-op without smarm-trace).
         #[cfg(feature = "smarm-trace")]
         crate::trace::flush();
+
+        // Propagate a root panic — last, after the full teardown above, so
+        // a `catch_unwind` around `run()` leaves the `Runtime` reusable.
+        // `Exit` and `Stopped` return normally: a cooperatively stopped
+        // root is not an error. `resume_unwind` re-raises the original
+        // payload; the throw-site hook output was suppressed in-actor, so
+        // the caller sees the payload message without the origin file:line.
+        if let Some(Outcome::Panic(payload)) = root_outcome {
+            // Surface the message: the throw-site hook output was suppressed
+            // in-actor, so without this a harness shows a bare FAILED.
+            let msg: Option<&str> = payload
+                .downcast_ref::<&'static str>()
+                .copied()
+                .or_else(|| payload.downcast_ref::<String>().map(String::as_str));
+            eprintln!(
+                "smarm: root actor panicked: {}",
+                msg.unwrap_or("<non-string panic payload>")
+            );
+            std::panic::resume_unwind(payload);
+        }
     }
 
     /// Snapshot of runtime statistics for introspection / tests.