test: deflake the suite — pumpEventQueue instead of Future.delayed(Duration.zero)

~173 `await Future<void>.delayed(Duration.zero)` async-settle waits across 25
test files yield the microtask queue exactly once; when an event→handler chain
needs more than one hop they lose the race under CI's parallel load, so the
failing set varied run to run. Replace with `await pumpEventQueue()` (the
deterministic drain already used elsewhere in the suite); rewired the shared
settle()/tick() helpers in one shot. menu_bar's toggle-close test gets a bounded
extra pump. Verified green under CI=true.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/kernel/reader_nav_test.dart

@@ -30,7 +30,7 @@ void main() {
   });
 
   // Let the broadcast bus deliver.
-  Future<void> tick() => Future<void>.delayed(Duration.zero);
+  Future<void> tick() => pumpEventQueue();
 
   test('starts empty', () {
     expect(nav.current, isNull);