feat(settings): Appearance category + custom-control escape hatch (T-452)

Add the one bespoke control the schema engine defers to. New
SettingsControlContribution routes a WidgetBuilder into a kernel
SettingsControlRegistry under a customId; a SettingsFieldKind.custom field
names that id, and the renderer draws the registered widget full-width
(label on top, no scope tag — the control owns its own apply + scope).

The theme-picker extension uses it: an Appearance category whose theme field
is custom, backed by AppearanceThemeControl — base-theme chips + a
high-contrast toggle that apply live through ThemeController (persisted by
theme_persistence). Reuses the shared theme_families helpers.

Tests: control registry (register/dup/unregister), the renderer's custom-field
path, and the Appearance contribution + live theme apply.

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

lib/kernel/src/facade.dart

@@ -35,6 +35,7 @@ import 'package:clide/kernel/src/scheduler.dart';
 import 'package:clide/kernel/src/secrets.dart';
 import 'package:clide/kernel/src/settings.dart';
 import 'package:clide/kernel/src/settings_registry.dart';
+import 'package:clide/kernel/src/settings_control_registry.dart';
 import 'package:clide/kernel/src/theme/controller.dart';
 import 'package:clide/kernel/src/theme/loader.dart';
 import 'package:clide/kernel/src/toolchain.dart';
@@ -83,6 +84,7 @@ class KernelServices {
     required this.toast,
     required this.logRing,
     required this.settingsRegistry,
+    required this.settingsControlRegistry,
   });
 
   final Logger log;
@@ -90,6 +92,9 @@ class KernelServices {
 
   /// Categories registered for the Settings panel (T-444).
   final SettingsRegistry settingsRegistry;
+
+  /// Bespoke widgets for custom settings fields (T-452).
+  final SettingsControlRegistry settingsControlRegistry;
   final DaemonBus events;
   final MessageBus messages;
 
@@ -161,6 +166,7 @@ class KernelServices {
     final settings = SettingsStore(appDir: appDir, onError: (m) => log.warn('settings', m));
     await settings.load();
     final settingsRegistry = SettingsRegistry();
+    final settingsControlRegistry = SettingsControlRegistry();
 
     final i18n = I18n(loader: i18nLoader, log: log, defaultLocale: defaultLocale, initialLocale: initialLocale, availableLocales: availableLocales);
     for (final ns in preloadNamespaces) {
@@ -245,6 +251,7 @@ class KernelServices {
       project: project,
       ipc: ipc,
       settingsRegistry: settingsRegistry,
+      settingsControlRegistry: settingsControlRegistry,
     );
 
     if (autoStartDaemonClient) {
@@ -256,6 +263,7 @@ class KernelServices {
       logRing: logRing,
       settings: settings,
       settingsRegistry: settingsRegistry,
+      settingsControlRegistry: settingsControlRegistry,
       events: events,
       messages: messages,
       filterStates: filterStates,