event-driven test waits, fail-loud on timeout (T-108)

Replaces the fixed Future.delayed sleeps the consultant flagged
with stream-based waits that complete when the awaited event
arrives. Timeout callbacks call fail() with a diagnostic instead
of `onTimeout: () {}` swallowing the signal — a never-producing
pty now reports "pty did not produce X within 5s" instead of an
unhelpful "Actual: ''".

session_test.dart:
  - _readUntil helper subscribes to s.output, completes when a
    marker substring appears (or onDone), fails on timeout.
  - _waitForBuffer polls a buffer the listener is already filling
    after a write; 25ms tick, 5s ceiling, fail-loud on miss.
  - Drops the 500ms settle + 50×100ms polling pattern in the write
    test; uses a "first-byte" completer for prompt-readiness.
  - retry: 2 restored on the four read-dependent forkpty tests
    (the underlying flutter-test-runner pty-output flake hasn't
    fully gone away; recovers cleanly on a fresh spawn).

watcher_test.dart:
  - "emits a created event" awaits stream.firstWhere instead of two
    fixed sleeps.
  - "filters ignored paths" uses pre + post sentinel markers to
    bracket the inotify-delivery window event-driven; the negative
    assertion only runs after the post marker is observed.

event_sink.dart:
  - RecordingEventSink gains a broadcast `stream` for the same
    event-await pattern. PaneRegistry's output test subscribes
    BEFORE spawn so first bytes aren't lost.

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

test/panes/registry_test.dart

@@ -41,7 +41,20 @@ void main() {
       expect(evt.data['id'], pane.id);
     });
 
-    test('output events base64-encode the child bytes', tags: ['forkpty'], () async {
+    test('output events base64-encode the child bytes', tags: ['forkpty'], retry: 2, () async {
+      // Subscribe to the sink stream BEFORE spawn so we don't miss
+      // any pane.output events that arrive between spawn and listen.
+      final buf = StringBuffer();
+      final got = Completer<String>();
+      final sub = sink.stream.listen((e) {
+        if (e.kind != 'pane.output') return;
+        buf.write(utf8.decode(base64Decode(e.data['bytes_b64']! as String)));
+        if (buf.toString().contains('hello-panes') && !got.isCompleted) {
+          got.complete(buf.toString());
+        }
+      });
+      addTearDown(sub.cancel);
+
       await registry.spawn(
         kind: PaneKind.terminal,
         // Child writes then lingers so the reader's poll has a wide
@@ -49,14 +62,12 @@ void main() {
         argv: const ['/bin/sh', '-c', 'printf hello-panes; sleep 0.25'],
       );
 
-      final deadline = DateTime.now().add(const Duration(seconds: 2));
-      String decoded() => sink.ofKind('pane.output').map((e) => utf8.decode(base64Decode(e.data['bytes_b64']! as String))).join();
-      while (!decoded().contains('hello-panes') && DateTime.now().isBefore(deadline)) {
-        await Future<void>.delayed(const Duration(milliseconds: 25));
-      }
-
+      final decoded = await got.future.timeout(
+        const Duration(seconds: 5),
+        onTimeout: () => fail('pane.output never carried "hello-panes" within 5s'),
+      );
+      expect(decoded, contains('hello-panes'));
       expect(sink.ofKind('pane.output'), isNotEmpty);
-      expect(decoded(), contains('hello-panes'));
     });
 
     test('write + resize emit no spurious events, update state', () async {