share a generous test I/O timeout constant

The flaky-gate fixes kept hand-tuning magic seconds in each real-I/O
test. Pull them into one Flutter-free constant — ioTimeout (20s) in
test/helpers/timeouts.dart, importable by both the dart-test (pty) and
flutter-test suites — and route the real-external-wait timeouts through
it: PTY output (session + registry), and fs-watcher events (timeout +
poll ceiling). Tune in one place instead of scattering durations.

The ipc socket round-trip timeouts (2s) are left as-is — they haven't
flaked and local sockets respond in ms; they can adopt the constant
later if needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/panes/registry_test.dart

@@ -13,6 +13,8 @@ import 'package:clide/clide.dart';
 import 'package:clide/src/panes/registry.dart';
 import 'package:test/test.dart';
 
+import '../helpers/timeouts.dart';
+
 void main() {
   if (!Platform.isLinux && !Platform.isMacOS) return;
 
@@ -63,8 +65,8 @@ void main() {
       );
 
       final decoded = await got.future.timeout(
-        const Duration(seconds: 5),
-        onTimeout: () => fail('pane.output never carried "hello-panes" within 5s'),
+        ioTimeout,
+        onTimeout: () => fail('pane.output never carried "hello-panes" within ${ioTimeout.inSeconds}s'),
       );
       expect(decoded, contains('hello-panes'));
       expect(sink.ofKind('pane.output'), isNotEmpty);