replace OS title bar with per-column 24px hats (D-057)

Hide GTK title bar via gtk_window_set_decorated(FALSE). Add
MethodChannel('clide/window') for drag/minimize/maximize/close
wired to GTK window functions. Dart WindowControls service wraps
the channel. Three per-column hats in RootLayout: left (macOS
traffic lights or plain drag), center (project > branch label),
right (minimize/maximize/close glyph buttons on Linux). Resolves
Q-006.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/window_controls.dart

@@ -0,0 +1,58 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+
+enum ChromeStyle { seam, prompt, inline }
+
+class WindowControls extends ChangeNotifier {
+  static const _channel = MethodChannel('clide/window');
+
+  ChromeStyle _style = ChromeStyle.seam;
+  ChromeStyle get style => _style;
+
+  void setStyle(ChromeStyle s) {
+    if (_style == s) return;
+    _style = s;
+    notifyListeners();
+  }
+
+  Future<void> startDrag() async {
+    try {
+      await _channel.invokeMethod('startDrag');
+    } on MissingPluginException {
+      // Web or unsupported platform — no-op.
+    }
+  }
+
+  Future<void> minimize() async {
+    try {
+      await _channel.invokeMethod('minimize');
+    } on MissingPluginException {
+      // no-op
+    }
+  }
+
+  Future<void> toggleMaximize() async {
+    try {
+      await _channel.invokeMethod('maximize');
+    } on MissingPluginException {
+      // no-op
+    }
+  }
+
+  Future<void> close() async {
+    try {
+      await _channel.invokeMethod('close');
+    } on MissingPluginException {
+      // no-op
+    }
+  }
+
+  Future<bool> isMaximized() async {
+    try {
+      final result = await _channel.invokeMethod<bool>('isMaximized');
+      return result ?? false;
+    } on MissingPluginException {
+      return false;
+    }
+  }
+}