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>

assets/keymaps/default.yaml

@@ -89,3 +89,9 @@ bindings:
     keys: [ctrl+minus, meta+minus]
   - intent: text.scaleReset
     keys: [ctrl+0, meta+0]
+
+  # -- File (application menu, T-48) -------------------------------------
+  - intent: command:file.openFolder
+    keys: [ctrl+o, meta+o]
+  - intent: command:file.newWindow
+    keys: [ctrl+shift+n, meta+shift+n]