test sweep: foundational systems — files / ipc errno / panes (T-91)

Three small foundational test additions:

- test/files/watcher_test.dart (new, 8 tests): FileChangeKind
  fromEvent across every FileSystemEvent.type + wire getter,
  FileChange.toJson, FileWatcher end-to-end (created event,
  ignored-path filtering, idempotent start, stop teardown).
- ignore_test: trailing /**, bare **, ? glob-pattern branches in
  the regex compiler.
- path_safety_test: PathOutsideRoot.toString embeds the three
  fields.
- ipc/errno_mapping_test: ENOTDIR / ENOMEM / EAGAIN branches in
  errnoToIpcError that weren't previously hit.
- panes/registry_test: RecordingEventSink.ofSubsystem filter.

Coverage: src/files/watcher.dart 10/36 -> ~ all; ignore.dart +
path_safety.dart residuals closed; ipc/errno_mapping.dart 3 added
branches; panes/event_sink.dart 100%.

Total coverage 90.53% -> 90.99%.

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

test/panes/registry_test.dart

@@ -103,4 +103,17 @@ void main() {
       expect(pane.toJson()['kind'], 'claude');
     });
   });
+
+  group('RecordingEventSink filters', () {
+    test('ofSubsystem narrows events to a single subsystem', () {
+      final s = RecordingEventSink();
+      final ts = DateTime.now().toUtc();
+      s.emit(IpcEvent(subsystem: 'pane', kind: 'spawned', timestamp: ts, data: const {}));
+      s.emit(IpcEvent(subsystem: 'git', kind: 'changed', timestamp: ts, data: const {}));
+      s.emit(IpcEvent(subsystem: 'pane', kind: 'closed', timestamp: ts, data: const {}));
+      expect(s.ofSubsystem('pane'), hasLength(2));
+      expect(s.ofSubsystem('git'), hasLength(1));
+      expect(s.ofSubsystem('files'), isEmpty);
+    });
+  });
 }