feat(settings): schema-driven settings engine (T-448)

The core of the settings panel (epic T-444). Subsystems describe a category
as data — a SettingsCategory of carded SettingsSections of SettingsFields
(toggle / select / text / number / opens-external-file), each bound to a
SettingsStore key with help text, a default, and reset-to-default.

Registration is declarative: a new SettingsCategoryContribution carries the
category; the extension manager routes it into a new kernel SettingsRegistry
(exposed on KernelServices), which the panel reads via ClideKernel. Adding a
category is now pure data + a contribution — no widget code.

SettingsCategoryView renders a category into carded sections per ui-design
surface.md: panelHeader card fill, dividerColor border, inputs receding to
panelBackground; select reuses the anchored-overlay menu, text/number commit
on Enter or blur (numeric clamps to bounds). The modal panel now shows the
selected/first registered category, falling back to the empty state.

Tests: registry (sort / dedup / notify), contribution routing on activation,
renderer (render + toggle/select write-through + reset), modal-with-category.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/settings_ui/category_view_test.dart

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+import 'package:clide/builtin/settings_ui/settings_ui.dart';
+import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../../helpers/kernel_fixture.dart';
+import '../../helpers/widget_harness.dart';
+
+const _category = SettingsCategory(
+  id: 'demo',
+  title: 'Demo',
+  iconName: 'gear',
+  sections: [
+    SettingsSection(
+      label: 'Group',
+      fields: [
+        SettingsField(key: 'app.demo.flag', kind: SettingsFieldKind.toggle, label: 'Flag', defaultValue: false),
+        SettingsField(
+          key: 'app.demo.level',
+          kind: SettingsFieldKind.select,
+          label: 'Level',
+          defaultValue: 'info',
+          options: [SettingsOption(value: 'info', label: 'Info'), SettingsOption(value: 'debug', label: 'Debug')],
+        ),
+        SettingsField(key: 'app.demo.name', kind: SettingsFieldKind.text, label: 'Name', defaultValue: ''),
+        SettingsField(key: 'app.demo.size', kind: SettingsFieldKind.number, label: 'Size', defaultValue: 4, min: 1, max: 8),
+      ],
+    ),
+  ],
+);
+
+/// Tiny extension that registers [_category] via the contribution.
+class _DemoSettingsExt extends ClideExtension {
+  @override
+  String get id => 'test.demo-settings';
+  @override
+  String get title => 'Demo settings';
+  @override
+  String get version => '1.0.0';
+  @override
+  List<ContributionPoint> get contributions => const [SettingsCategoryContribution(id: 'demo', category: _category)];
+}
+
+Widget _bounded(Widget child) => SizedBox(width: 620, height: 520, child: child);
+
+Finder get _toggle => find.byWidgetPredicate((w) => w is Semantics && w.properties.checked != null);
+
+void main() {
+  late KernelFixture f;
+
+  setUp(() async {
+    f = await KernelFixture.create();
+  });
+  tearDown(() async => f.dispose());
+
+  group('SettingsCategoryContribution routing', () {
+    test('an activated extension registers its category in the registry', () async {
+      f.services.extensions.register(_DemoSettingsExt());
+      await f.services.extensions.activateAll();
+      expect(f.services.settingsRegistry.byId('demo')?.title, 'Demo');
+    });
+  });
+
+  group('SettingsCategoryView', () {
+    testWidgets('renders the section header, field labels, and current values', (tester) async {
+      await tester.pumpWidget(harness(f, _bounded(const SettingsCategoryView(category: _category))));
+      expect(find.text('GROUP'), findsOneWidget); // small-caps section header
+      expect(find.text('Flag'), findsOneWidget);
+      expect(find.text('Level'), findsOneWidget);
+      expect(find.text('Name'), findsOneWidget);
+      expect(find.text('Size'), findsOneWidget);
+      // Select shows the default option's label when the key is unset.
+      expect(find.text('Info'), findsOneWidget);
+    });
+
+    testWidgets('tapping the toggle writes the flipped value to the store', (tester) async {
+      await tester.pumpWidget(harness(f, _bounded(const SettingsCategoryView(category: _category))));
+      expect(_toggle, findsOneWidget);
+      await tester.tap(_toggle);
+      await tester.pump();
+      expect(f.services.settings.get<bool>('app.demo.flag'), isTrue);
+    });
+
+    testWidgets('picking a select option writes it to the store', (tester) async {
+      await tester.pumpWidget(harness(f, _bounded(const SettingsCategoryView(category: _category))));
+      await tester.tap(find.bySemanticsLabel(RegExp(r'^Level:')));
+      await tester.pump();
+      await tester.tap(find.text('Debug'));
+      await tester.pump();
+      expect(f.services.settings.get<String>('app.demo.level'), 'debug');
+    });
+
+    testWidgets('reset control appears for a non-default value and restores the default', (tester) async {
+      await tester.runAsync(() => f.services.settings.set('app.demo.flag', true));
+      await tester.pumpWidget(harness(f, _bounded(const SettingsCategoryView(category: _category))));
+      final reset = find.bySemanticsLabel('Reset to default');
+      expect(reset, findsOneWidget);
+      await tester.tap(reset);
+      await tester.pump();
+      expect(f.services.settings.get<bool>('app.demo.flag'), isFalse);
+    });
+  });
+
+  group('SettingsModal with a registered category', () {
+    testWidgets('renders the category instead of the empty state', (tester) async {
+      f.services.settingsRegistry.register(_category);
+      await tester.pumpWidget(harness(f, SettingsModal(onDismiss: () {})));
+      expect(find.text('Flag'), findsOneWidget);
+      expect(find.text('No settings categories are registered yet.'), findsNothing);
+    });
+  });
+}

test/kernel/src/settings_registry_test.dart

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+import 'package:clide/kernel/kernel.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+SettingsCategory _cat(String id, {String? title, int priority = 0}) =>
+    SettingsCategory(id: id, title: title ?? id, priority: priority, sections: const []);
+
+void main() {
+  group('SettingsRegistry', () {
+    test('register exposes categories sorted by (priority, title)', () {
+      final r = SettingsRegistry();
+      r.register(_cat('b', title: 'Beta', priority: 10));
+      r.register(_cat('a', title: 'Alpha', priority: 10));
+      r.register(_cat('z', title: 'Zeta', priority: 0));
+
+      expect(r.categories.map((c) => c.id).toList(), ['z', 'a', 'b']);
+    });
+
+    test('byId resolves a registered category', () {
+      final r = SettingsRegistry()..register(_cat('editor', title: 'Editor'));
+      expect(r.byId('editor')?.title, 'Editor');
+      expect(r.byId('missing'), isNull);
+    });
+
+    test('duplicate id throws (rolls activation back)', () {
+      final r = SettingsRegistry()..register(_cat('dup'));
+      expect(() => r.register(_cat('dup')), throwsStateError);
+    });
+
+    test('notifies on register and unregister', () {
+      final r = SettingsRegistry();
+      var n = 0;
+      r.addListener(() => n++);
+      r.register(_cat('x'));
+      expect(n, 1);
+      r.unregister('x');
+      expect(n, 2);
+      expect(r.categories, isEmpty);
+    });
+
+    test('unregister of an unknown id is a no-op (no notify)', () {
+      final r = SettingsRegistry();
+      var n = 0;
+      r.addListener(() => n++);
+      r.unregister('nope');
+      expect(n, 0);
+    });
+  });
+}