lift text-zoom into kernel, surface it in the palette (T-114)

Workspace text-zoom (Ctrl +/-/0) was local state on _RootShellState,
reachable only via the keymap intent path. Lifted to a kernel TextZoom
ChangeNotifier so the new `view.zoomIn/Out/Reset` palette commands
mutate the same number the keymap does — closing T-114's "discoverable
in the palette" item.

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

test/kernel/src/text_zoom_test.dart

@@ -0,0 +1,60 @@
+import 'package:clide/kernel/src/text_zoom.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('TextZoom', () {
+    test('starts at 1.0', () {
+      expect(TextZoom().scale, 1.0);
+    });
+
+    test('increase adds one step, notifies listeners', () {
+      final z = TextZoom();
+      var calls = 0;
+      z.addListener(() => calls++);
+      z.increase();
+      expect(z.scale, closeTo(1.0 + TextZoom.stepScale, 1e-9));
+      expect(calls, 1);
+    });
+
+    test('decrease subtracts one step', () {
+      final z = TextZoom();
+      z.decrease();
+      expect(z.scale, closeTo(1.0 - TextZoom.stepScale, 1e-9));
+    });
+
+    test('reset jumps back to 1.0', () {
+      final z = TextZoom()..increase()..increase();
+      expect(z.scale, isNot(1.0));
+      z.reset();
+      expect(z.scale, 1.0);
+    });
+
+    test('clamps at minScale', () {
+      final z = TextZoom();
+      for (var i = 0; i < 100; i++) {
+        z.decrease();
+      }
+      expect(z.scale, TextZoom.minScale);
+    });
+
+    test('clamps at maxScale', () {
+      final z = TextZoom();
+      for (var i = 0; i < 100; i++) {
+        z.increase();
+      }
+      expect(z.scale, TextZoom.maxScale);
+    });
+
+    test('no-op increment does not notify', () {
+      final z = TextZoom();
+      for (var i = 0; i < 100; i++) {
+        z.increase();
+      }
+      // Already at max — the next increase shouldn't fire.
+      var calls = 0;
+      z.addListener(() => calls++);
+      z.increase();
+      expect(calls, 0);
+    });
+  });
+}