fix(io): deregister a stopped actor's fd wait on the unwind path

A stopped actor unwinding out of wait_fd's park leaked its waiters entry
and the kernel-side EPOLLONESHOT registration; the stale entry then failed
every future wait_*() on that fd with AlreadyExists (the defensive bare DEL
in epoll_register sits behind the contains_key check, so it never ran).

Fix where the invariant breaks: a drop guard in wait_fd, armed after a
successful register and forgotten on the normal wake (where FdReady already
removed + DEL'd). On unwind it cleans up iff the entry is still this wait's
(pid, epoch) — an entry consumed by a racing FdReady means the fd may carry
another actor's fresh registration, which must be left alone. Closes the
v0.2 fd-hygiene TODO.

src/scheduler.rs

@@ -396,7 +396,39 @@ fn wait_fd(fd: std::os::fd::RawFd, readable: bool, writable: bool) -> std::io::R
         let mut io = inner.io.lock().unwrap();
         io.as_mut().expect("io thread not started").epoll_register(fd, me, epoch, readable, writable)
     })?;
+
+    // If a terminal stop unwinds us out of the park below, the registration
+    // must not outlive us: a stale `waiters` entry fails every future
+    // `wait_*` on this fd with AlreadyExists, and the kernel-side ADD leaks
+    // until the fd happens to be reused. Clean up iff the entry is still
+    // ours — a `FdReady` racing the stop may have consumed it already (it
+    // removes + DELs under the io lock), after which the fd may even carry
+    // ANOTHER actor's fresh registration; in that case touch nothing.
+    struct Dereg {
+        fd: std::os::fd::RawFd,
+        me: Pid,
+        epoch: u32,
+    }
+    impl Drop for Dereg {
+        fn drop(&mut self) {
+            with_runtime(|inner| {
+                let mut io = inner.io.lock().unwrap();
+                if let Some(io) = io.as_mut() {
+                    if io.waiters.get(&self.fd) == Some(&(self.me, self.epoch)) {
+                        io.waiters.remove(&self.fd);
+                        io.epoll_deregister(self.fd);
+                    }
+                }
+            });
+        }
+    }
+    let guard = Dereg { fd, me, epoch };
     park_current();
+    // Normal wake: the FdReady path removed the entry and DEL'd the fd
+    // before the unpark, so the guard's check would be a guaranteed no-op —
+    // skip the io lock on the hot path. (Dereg owns nothing; forget leaks
+    // no resource.)
+    std::mem::forget(guard);
     Ok(())
 }