video: desktop controls, shared scrubber, lifecycle guard and first-frame gate; zoomable and adapter tweaks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/video/first_frame_gate_test.dart

@@ -0,0 +1,43 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:leaf_widgets/leaf_widgets.dart';
+
+void main() {
+  test('immediate entry while slots are free', () async {
+    final gate = FirstFrameGate(maxConcurrent: 2);
+    await gate.enter();
+    await gate.enter();
+    expect(gate.activeCount, 2);
+    expect(gate.waitingCount, 0);
+    gate.exit();
+    gate.exit();
+    expect(gate.activeCount, 0);
+  });
+
+  test('third entrant waits, exit hands the slot over', () async {
+    final gate = FirstFrameGate(maxConcurrent: 2);
+    await gate.enter();
+    await gate.enter();
+
+    var thirdIn = false;
+    final pending = gate.enter().then((_) => thirdIn = true);
+    await Future<void>.delayed(Duration.zero);
+    expect(thirdIn, isFalse);
+    expect(gate.waitingCount, 1);
+
+    gate.exit();
+    await pending;
+    expect(thirdIn, isTrue);
+    expect(gate.activeCount, 2);
+
+    gate.exit();
+    gate.exit();
+    expect(gate.activeCount, 0);
+  });
+
+  test('exit without enter never goes negative', () {
+    final gate = FirstFrameGate(maxConcurrent: 1);
+    gate.exit();
+    expect(gate.activeCount, 0);
+  });
+}