fix dead default keymap from undefined focus intents

default.yaml bound tab/shift+tab to focus.next/focus.previous, intent
ids absent from builtinIntents. parseIntentId returns null, so
KeymapLayer.fromYaml throws — and KeymapService.load catches that and
sets _preset = null. The whole default preset was silently dropped at
boot: palette, quick-open, find-in-files, and zoom bindings never fired.

It went unnoticed because every keymap_service_test injects a synthetic
bundle; the shipped asset was never parsed in a test. Add focus.next ->
NextFocusIntent and focus.previous -> PreviousFocusIntent (Flutter-
provided, for correct Tab widget traversal), and a test that reads every
real assets/keymaps/*.yaml through the loader so a future typo fails CI
instead of disabling the keymap.

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

lib/kernel/src/keymap/intents.dart

@@ -119,6 +119,12 @@ class InvokeCommandIntent extends Intent {
 final Map<String, Intent Function()> builtinIntents = {
   'activate': () => const ActivateIntent(),
   'dismiss': () => const DismissIntent(),
+  // Widget-level focus traversal (Tab / Shift+Tab). These are
+  // Flutter-provided, like activate/dismiss — bound by id so the
+  // default preset integrates with the framework's focus system. They
+  // are distinct from the panel-to-panel cycling below (F6).
+  'focus.next': () => const NextFocusIntent(),
+  'focus.previous': () => const PreviousFocusIntent(),
   'focus.nextPanel': () => const FocusNextPanelIntent(),
   'focus.previousPanel': () => const FocusPreviousPanelIntent(),
   'palette.open': () => const PaletteOpenIntent(),