Stop cancelled remote queries without blocking the selector

RemoteQueryPool.cancel() terminated the SSH process and then waited up
to 250 ms for it, on the curses thread. Ctrl+R re-enqueues every host,
so a refresh of six hosts could freeze the popup for over a second.

Only send SIGTERM when cancelling; reap the process on later polls,
escalating to SIGKILL after 250 ms and closing its pipes once it has
exited. close() still waits, since the popup is exiting anyway.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test_remotes.py

@@ -282,6 +282,33 @@ class RemoteSshTests(unittest.TestCase):
 
         self.assertEqual(decode_remote_output("错误".encode("gbk")), "错误")
 
+    def test_cancel_does_not_block_and_reaps_the_process_later(self) -> None:
+        with tempfile.TemporaryDirectory() as directory:
+            slow_ssh = Path(directory) / "ssh"
+            # Ignore SIGTERM so the pool has to escalate to SIGKILL.
+            slow_ssh.write_text("#!/bin/sh\ntrap '' TERM\nsleep 5 & wait\n", encoding="utf-8")
+            slow_ssh.chmod(0o755)
+            pool = RemoteQueryPool(str(slow_ssh), query_timeout=5)
+            self.addCleanup(pool.close)
+            pool.enqueue(RemoteEntry("slow"))
+            time.sleep(0.05)
+            process = pool._active["slow"].process  # type: ignore[attr-defined]
+
+            started = time.monotonic()
+            pool.cancel("slow")
+            self.assertLess(time.monotonic() - started, 0.1)
+            self.assertFalse(pool.is_loading("slow"))
+
+            deadline = time.monotonic() + 3
+            while process.poll() is None and time.monotonic() < deadline:
+                pool.poll()
+                time.sleep(0.02)
+            self.assertIsNotNone(process.poll())
+            pool.poll()
+            self.assertEqual(pool._stopping, [])  # type: ignore[attr-defined]
+            assert process.stdout is not None
+            self.assertTrue(process.stdout.closed)
+
     def test_query_pool_times_out_without_blocking_other_results(self) -> None:
         with tempfile.TemporaryDirectory() as directory:
             slow_ssh = Path(directory) / "ssh"