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/ipc/errno_mapping_test.dart

@@ -51,5 +51,24 @@ void main() {
       expect(err.message, contains('kernel exploded'));
       expect(err.message, isNot(contains('errno=999')));
     });
+
+    test('ENOTDIR → user_error', () {
+      final e = errnoToIpcError(errno: PosixErrno.enotdir, op: 'files.ls', target: '/x');
+      expect(e.kind, IpcErrorKind.userError);
+      expect(e.message, contains('not a directory'));
+    });
+
+    test('ENOMEM → tool_error', () {
+      final e = errnoToIpcError(errno: PosixErrno.enomem, op: 'pty.spawn');
+      expect(e.kind, IpcErrorKind.toolError);
+      expect(e.message, contains('out of memory'));
+    });
+
+    test('EAGAIN → tool_error with retry hint', () {
+      final e = errnoToIpcError(errno: PosixErrno.eagain, op: 'pty.read');
+      expect(e.kind, IpcErrorKind.toolError);
+      expect(e.message, contains('temporarily unavailable'));
+      expect(e.hint, contains('retry'));
+    });
   });
 }