scheduler: spawn_monitor / spawn_monitor_with — monitor registered on the child's slot before publish, so the Down always carries the real reason (spawn-then-monitor could race to NoProc); tests; channel test uses it

src/runtime.rs

@@ -1764,6 +1764,7 @@ pub(crate) fn install_actor(
     stack: crate::stack::Stack,
     supervisor: Pid,
     closure: Closure,
+    monitor: Option<(crate::monitor::MonitorId, crate::channel::Sender<crate::monitor::Down>)>,
 ) -> Pid {
     let slot = &inner.slots[idx as usize];
     let gen = slot.generation(); // stable: we own the vacant slot via the free list
@@ -1791,6 +1792,12 @@ pub(crate) fn install_actor(
         cold.outstanding_handles = 1;
         cold.outcome = None;
         cold.pending_io_result = None;
+        // `spawn_monitor`: register before publish, so no scheduler can run
+        // (and finalize) the child before its monitor exists. Same slot as a
+        // `monitor()` registration; the send-from-finalize path is unchanged.
+        if let Some(m) = monitor {
+            cold.monitors.push(m);
+        }
     }
     slot.sp.store(sp, Ordering::Relaxed);
     // RFC 019: a fresh incarnation starts with its high-water at the fresh