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/files/path_safety_test.dart

@@ -58,5 +58,10 @@ void main() {
         if (twin.existsSync()) twin.deleteSync(recursive: true);
       }
     });
+
+    test('PathOutsideRoot.toString embeds requested + resolved + root', () {
+      final e = PathOutsideRoot('r', '/abs', '/root');
+      expect(e.toString(), allOf(contains('r'), contains('/abs'), contains('/root')));
+    });
   });
 }