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/ignore_test.dart

@@ -76,5 +76,23 @@ void main() {
       expect(s.isIgnored('docs/a.draft.md', isDirectory: false), isTrue);
       expect(s.isIgnored('other/a.draft.md', isDirectory: false), isFalse);
     });
+
+    test('trailing /** matches every descendant', () {
+      final s = IgnoreSet.parse(['build/**']);
+      expect(s.isIgnored('build/a/b/c.txt', isDirectory: false), isTrue);
+      expect(s.isIgnored('src/a.txt', isDirectory: false), isFalse);
+    });
+
+    test('bare ** (not adjacent to /) matches across path separators', () {
+      final s = IgnoreSet.parse(['a**z']);
+      expect(s.isIgnored('axyz', isDirectory: false), isTrue);
+      expect(s.isIgnored('amiddlez', isDirectory: false), isTrue);
+    });
+
+    test('? matches exactly one non-slash character', () {
+      final s = IgnoreSet.parse(['a?c']);
+      expect(s.isIgnored('abc', isDirectory: false), isTrue);
+      expect(s.isIgnored('a/c', isDirectory: false), isFalse);
+    });
   });
 }