add Zed-style application menu bar: File / View / Help (T-48)

A custom in-window menu bar in the hat (no native menu, D-7), built from
the command registry so it stays in sync and satisfies D-6 parity.

- Menu model + hybrid resolver (menu_model.dart): a curated File/View/Help
  tree where a MenuAutoFill node sweeps in unplaced view.* commands; titles
  + keybindings come from the registry/keymap; unregistered or
  enabledWhen-false items render disabled (greyed), never hidden.
- Widgets: MenuBar row in the hat (chrome tokens), anchored MenuDropdown
  overlay (dropdown tokens), two-column MenuItemRow with inline keybinding.
- Full keyboard: Alt+mnemonic opens (hook in _RootShell._onKey), arrows
  navigate, Enter activates, Esc closes, Left/Right switch menus.
- Commands: file.openFolder / file.newWindow / file.closeWorkspace /
  help.about, registered by MenuBarExtension(services:). File logic lifted
  out of the project switcher into FileActions (one source of truth; the
  switcher now dispatches the commands). Ctrl+O / Ctrl+Shift+N are now real
  keybindings in default.yaml.
- Help → About: version/commit/date/repo from build-info + the bundled
  dependency licenses parsed from assets/licenses.yaml.

Edit/Selection menus are deferred to T-271/T-272 (need focused-surface
command routing).

Tests: resolver + controller + licenses parse (pure); menu-bar widget
(open/close/execute/disabled/Esc/arrow/Enter/Left-Right); FileActions +
Open dialog; app-level Alt+F, non-repo dialog, and closeWorkspace.

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

test/builtin/menubar/licenses_loader_test.dart

@@ -0,0 +1,64 @@
+/// Pure parse tests for the About-dialog licenses manifest (T-48).
+library;
+
+import 'package:clide/builtin/menubar/src/licenses_loader.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('parses self + dependency entries', () {
+    const yaml = '''
+self:
+  name: clide
+  version: "2.1.0"
+  license: MIT
+dependencies:
+  - name: Foo
+    version: "1.0"
+    license: MIT
+  - name: Bar
+    version: "2.0"
+    license: OFL-1.1
+''';
+    final m = parseLicenses(yaml);
+    expect(m.self.name, 'clide');
+    expect(m.self.version, '2.1.0');
+    expect(m.self.license, 'MIT');
+    expect(m.dependencies, hasLength(2));
+    expect(m.dependencies[0].name, 'Foo');
+    expect(m.dependencies[1].license, 'OFL-1.1');
+  });
+
+  test('missing fields degrade to a dash rather than throwing', () {
+    final m = parseLicenses('dependencies:\n  - name: X\n');
+    expect(m.self.name, '—');
+    expect(m.dependencies.single.version, '—');
+    expect(m.dependencies.single.license, '—');
+  });
+
+  test('empty document yields an empty manifest', () {
+    final m = parseLicenses('');
+    expect(m.self.name, '—');
+    expect(m.dependencies, isEmpty);
+  });
+
+  test('the bundled assets/licenses.yaml is real and non-trivial', () {
+    final m = parseLicenses(_bundled);
+    expect(m.self.name, 'clide');
+    expect(m.dependencies, isNotEmpty);
+  });
+}
+
+// A trimmed copy of the real manifest shape, to assert parseLicenses handles
+// the comment-heavy, quoted-value document the app ships.
+const _bundled = '''
+schema_version: 1
+self:
+  name: clide
+  version: "2.1.0"
+  license: MIT
+dependencies:
+  - name: JetBrains Mono
+    kind: font
+    version: "2.304"
+    license: OFL-1.1
+''';