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/kernel/src/files.dart

@@ -0,0 +1,61 @@
+import 'dart:async';
+
+import 'package:clide_app/kernel/src/events/bus.dart';
+import 'package:clide_app/kernel/src/events/types.dart';
+import 'package:clide_app/kernel/src/panels/slot_id.dart';
+import 'package:flutter/foundation.dart';
+
+class FilesDropped extends ClideEvent {
+  const FilesDropped({required this.paths, required this.slot});
+
+  final List<String> paths;
+  final SlotId slot;
+
+  @override
+  String get subsystem => 'files';
+  @override
+  String get kind => 'dropped';
+  @override
+  Map<String, Object?> payload() => {
+        'paths': paths,
+        'slot': slot.value,
+      };
+}
+
+/// Tier-0 stub for file pickers and drop targets.
+///
+/// Flutter desktop has no native picker API without a dep; rather than
+/// add one now, pickOpen/pickSave/pickDirectory throw UnimplementedError
+/// and the drop target is a no-op until we wire it through the
+/// platform channel. This lets the rest of the kernel compile and makes
+/// the service surface real.
+class FileServices {
+  FileServices(this._events);
+  final EventBus _events;
+
+  Future<List<String>> pickOpen({
+    List<String> extensions = const [],
+    bool multiple = false,
+  }) async {
+    throw UnimplementedError('pickOpen — wired in a later tier');
+  }
+
+  Future<String?> pickSave({
+    String? defaultName,
+    List<String> extensions = const [],
+  }) async {
+    throw UnimplementedError('pickSave — wired in a later tier');
+  }
+
+  Future<String?> pickDirectory() async {
+    throw UnimplementedError('pickDirectory — wired in a later tier');
+  }
+
+  /// Invoked by the platform drop-target wiring when files land on a
+  /// slot. Emits a [FilesDropped] event; the slot-owning extension
+  /// subscribes.
+  @visibleForTesting
+  void notifyDropped({required List<String> paths, required SlotId slot}) {
+    _events.emit(FilesDropped(paths: paths, slot: slot));
+  }
+}