scaffold Flutter app — kernel, extensions, widgets, Tier 0 built-ins

First real content under app/. Lays the whole Tier 0 foundation in one
commit because the pieces depend on each other circularly (kernel →
extension → widgets → built-ins all reference types from the layer
below); splitting would leave intermediate commits that don't compile.

Key shapes:

  * bare WidgetsApp root — no Material, no Cupertino, no Scaffold.
    ClideTheme InheritedWidget is the only source of tokens.
  * ClideKernel InheritedWidget aggregates 18 services (settings,
    project, extensions, theme, panels, events, ipc, commands +
    palette + keybindings, clipboard, files, notify, dialog, tray,
    secrets, os, net, focus, log, i18n). ExtensionContext exposes
    them through a stable interface.
  * ClideExtension + sealed ContributionPoint hierarchy (Tab,
    StatusItem, Toolbar, Command, TrayItem, LayoutPreset). One
    manifest ships N contributions into kernel slots.
  * Three-tier theme pipeline: palette (named colors) → semantic
    roles → ~60 surface tokens. Defaults at each layer so legacy
    palette-only themes produce a complete SurfaceTokens.
  * A11y baked in from day one — every interactive primitive wraps
    in Semantics(label:, hint:, button:); ensureSemantics() at boot;
    theme/contrast.dart helper exposes token pairs for the WCAG
    gate in the a11y test suite.
  * i18n ported from fframe's L10n pattern (text-driven, namespaced
    JSON catalogs, caller-supplied placeholders) with a proper
    locale fallback chain (exact → language → default → placeholder)
    fframe lacks.
  * Four Tier-0 built-ins live (default-layout, welcome, ipc-status,
    theme-picker) plus 17 id-reserving stubs so later tiers fill in
    without rename churn.

.gitignore extended to cover app/ sub-package artefacts and the
Playwright harness scratch dirs.

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

app/lib/builtin/default_layout/src/extension.dart

@@ -0,0 +1,53 @@
+import 'package:clide/clide.dart';
+import 'package:clide_app/extension/extension.dart';
+import 'package:clide_app/kernel/kernel.dart';
+
+class DefaultLayoutExtension extends ClideExtension {
+  @override
+  String get id => 'builtin.default-layout';
+  @override
+  String get title => 'Default layout';
+  @override
+  String get version => '0.1.0';
+
+  LayoutPresetContribution? _preset;
+  ClideExtensionContext? _ctx;
+
+  @override
+  List<ContributionPoint> get contributions => [
+        _preset ?? classicPreset(),
+        CommandContribution(
+          id: 'layout.reset',
+          command: 'layout.reset',
+          title: 'Layout: Reset to Classic',
+          run: _reset,
+        ),
+      ];
+
+  @override
+  Future<void> activate(ClideExtensionContext ctx) async {
+    _ctx = ctx;
+    _preset = classicPreset();
+    // Register the preset's slots into the panel registry and apply
+    // the arrangement so every SlotHost has something to render.
+    ctx.arrangement.registerSlotsInto(ctx.panels, _preset!);
+    ctx.arrangement.applyPreset(_preset!);
+  }
+
+  Future<IpcResponse> _reset(List<String> args) async {
+    final preset = _preset;
+    final ctx = _ctx;
+    if (preset == null || ctx == null) {
+      return IpcResponse.err(
+        id: '',
+        error: IpcError(
+          code: IpcExitCode.toolError,
+          kind: IpcErrorKind.toolError,
+          message: 'default-layout not activated',
+        ),
+      );
+    }
+    ctx.arrangement.applyPreset(preset);
+    return IpcResponse.ok(id: '', data: {'preset': preset.id});
+  }
+}