test sweep: cover base/disposable.dart (T-91)

Three pure-Dart tests covering the Disposable mixin: the disposed
getter flipping after dispose(), onDisposed firing once, and
register propagating dispose to child disposables.

Coverage: disposable.dart 12/17 -> 17/17. Total 71.20% -> 71.23%.

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

test/terminal/disposable_test.dart

@@ -0,0 +1,37 @@
+/// Unit tests for `lib/src/terminal/src/base/disposable.dart` —
+/// covers the `disposed` flag, the `onDisposed` event, and registering
+/// child disposables.
+library;
+
+import 'package:clide/src/terminal/src/base/disposable.dart';
+import 'package:test/test.dart';
+
+class _D with Disposable {}
+
+void main() {
+  group('Disposable mixin', () {
+    test('disposed flips from false to true after dispose()', () {
+      final d = _D();
+      expect(d.disposed, isFalse);
+      d.dispose();
+      expect(d.disposed, isTrue);
+    });
+
+    test('onDisposed fires once when dispose() runs', () {
+      final d = _D();
+      var fired = 0;
+      d.onDisposed((_) => fired++);
+      d.dispose();
+      expect(fired, 1);
+    });
+
+    test('register propagates dispose to child disposables', () {
+      final parent = _D();
+      final child = _D();
+      parent.register(child);
+      expect(child.disposed, isFalse);
+      parent.dispose();
+      expect(child.disposed, isTrue);
+    });
+  });
+}