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>

lib/kernel/src/settings_registry.dart

@@ -0,0 +1,37 @@
+import 'package:clide/kernel/src/settings_schema.dart';
+import 'package:flutter/foundation.dart';
+
+/// Holds the [SettingsCategory] schemas subsystems register against the kernel
+/// (via `SettingsCategoryContribution`, routed by the extension manager). The
+/// settings panel reads this to build its rail + panels (T-447/T-448) and
+/// rebuilds when the set changes.
+class SettingsRegistry extends ChangeNotifier {
+  final Map<String, SettingsCategory> _byId = <String, SettingsCategory>{};
+
+  /// Registered categories, sorted by (priority, then case-insensitive title).
+  List<SettingsCategory> get categories {
+    final list = _byId.values.toList()
+      ..sort((a, b) {
+        final p = a.priority.compareTo(b.priority);
+        return p != 0 ? p : a.title.toLowerCase().compareTo(b.title.toLowerCase());
+      });
+    return List.unmodifiable(list);
+  }
+
+  SettingsCategory? byId(String id) => _byId[id];
+
+  /// Register a category. Throws on a duplicate id — a collision is a wiring
+  /// bug that should roll the contributing extension's activation back, the
+  /// same way duplicate command/slot ids do.
+  void register(SettingsCategory category) {
+    if (_byId.containsKey(category.id)) {
+      throw StateError('duplicate settings category id: ${category.id}');
+    }
+    _byId[category.id] = category;
+    notifyListeners();
+  }
+
+  void unregister(String id) {
+    if (_byId.remove(id) != null) notifyListeners();
+  }
+}