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/extensions_manager.dart

@@ -26,6 +26,7 @@ import 'package:clide/kernel/src/project.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/tray.dart';
 import 'package:flutter/foundation.dart';
@@ -57,6 +58,7 @@ class ExtensionManager extends ChangeNotifier {
     required this.project,
     required this.ipc,
     required this.settingsRegistry,
+    required this.settingsControlRegistry,
   });
 
   final Logger log;
@@ -84,6 +86,7 @@ class ExtensionManager extends ChangeNotifier {
   final ProjectManager project;
   final DaemonClient ipc;
   final SettingsRegistry settingsRegistry;
+  final SettingsControlRegistry settingsControlRegistry;
 
   final Map<String, ClideExtension> _known = {};
   final Set<String> _activated = {};
@@ -262,6 +265,8 @@ class ExtensionManager extends ChangeNotifier {
       case SettingsCategoryContribution s:
         // register() throws on a duplicate id, rolling activation back.
         settingsRegistry.register(s.category);
+      case SettingsControlContribution s:
+        settingsControlRegistry.register(s.customId, s.builder);
     }
   }
 
@@ -284,6 +289,8 @@ class ExtensionManager extends ChangeNotifier {
         break;
       case SettingsCategoryContribution s:
         settingsRegistry.unregister(s.category.id);
+      case SettingsControlContribution s:
+        settingsControlRegistry.unregister(s.customId);
     }
   }
 

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,

lib/kernel/src/settings_control_registry.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/widgets.dart';
+
+/// Holds the bespoke widgets that draw [SettingsFieldKind.custom] fields
+/// (T-452). A subsystem registers a builder under a `customId` (via
+/// `SettingsControlContribution`, routed by the extension manager); the
+/// settings renderer looks it up when it meets a custom field.
+///
+/// Controls register at activation, before any settings modal opens, so this
+/// is a plain registry — no change notification needed.
+class SettingsControlRegistry {
+  final Map<String, WidgetBuilder> _byId = <String, WidgetBuilder>{};
+
+  /// Register the [builder] for [customId]. Throws on a duplicate id so a
+  /// collision rolls the contributing extension's activation back.
+  void register(String customId, WidgetBuilder builder) {
+    if (_byId.containsKey(customId)) {
+      throw StateError('duplicate settings control id: $customId');
+    }
+    _byId[customId] = builder;
+  }
+
+  void unregister(String customId) => _byId.remove(customId);
+
+  /// The builder for [customId], or null when none is registered.
+  WidgetBuilder? builderFor(String customId) => _byId[customId];
+}

lib/kernel/src/settings_schema.dart

@@ -24,6 +24,12 @@ enum SettingsFieldKind {
   /// of editing a value inline — the action is a command id, keeping the
   /// schema widget-free.
   file,
+
+  /// A bespoke control rendered by a widget the owning subsystem registers
+  /// (via `SettingsControlContribution`) under [SettingsField.customId]. Keeps
+  /// the schema widget-free while allowing the one-off controls the generic
+  /// kinds can't express (e.g. the Appearance theme picker).
+  custom,
 }
 
 /// One choice in a [SettingsFieldKind.select] field.
@@ -53,6 +59,7 @@ class SettingsField {
     this.max,
     this.fileCommand,
     this.applyCommandPrefix,
+    this.customId,
   });
 
   final String key;
@@ -81,6 +88,10 @@ class SettingsField {
   /// persists). The current value is still read from [key], so the scope tag
   /// and selection still work. Example: `'keymap.preset.'` → `keymap.preset.vim`.
   final String? applyCommandPrefix;
+
+  /// For [SettingsFieldKind.custom]: the id the renderer looks up in the
+  /// `SettingsControlRegistry` to find the widget that draws this field.
+  final String? customId;
 }
 
 /// A carded group of fields (surface.md "sectioned cards"). [label] is the