vim normal-mode navigation in non-editor panes (T-406)

The structural T-403 child: make vim normal mode mean navigation in panes
that were mouse-only. The passive global key path can't run multi-chord
sequences (D-82), so each pane hosts its own SequenceMatcher — factored into
a reusable PaneKeyNav that resolves the live keymap and dispatches nav.*
intents while a pane holds focus under the vim preset.

- nav.* intents (down/up/pageDown/pageUp/top/bottom/expandOrRight/
  collapseOrLeft/activate) — preset-neutral; vim.yaml binds j/k/ctrl+d/ctrl+u/
  gg/G/l/h/[o,enter] under `vim.normal && !editor.focused`.
- The editor publishes an `editor.focused` scope flag from its focus node, so
  the same keys stay buffer motions while the editor is focused and become nav
  when a pane is — resolved by file order + the guard (no change to the editor
  motion bindings).
- File tree: a flattened visible-index selection cursor in FileTreeController
  (j/k move, h collapse-or-out, l expand-or-into, o/enter open), with a focus
  ring + scroll-into-view.
- Conversation: j/k line-scroll, ctrl+d/u half-page, gg top, G bottom — G
  re-arms follow-tail.

Foundation for T-404/T-405/T-407, which build on the per-pane matcher and the
editor.focused guard. Git panel + ticket board list nav deferred to a
follow-up (the ticket says lists can trail). Tests: keymap resolution under
both scopes, PaneKeyNav dispatch, the controller selection model, and
end-to-end key-driven nav in both panes.

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

lib/kernel/kernel.dart

@@ -28,6 +28,7 @@ export 'src/keymap/key_chord.dart';
 export 'src/keymap/keymap.dart';
 export 'src/keymap/keymap_service.dart';
 export 'src/keymap/modifier_tap.dart';
+export 'src/keymap/pane_key_nav.dart';
 export 'src/keymap/sequence_matcher.dart';
 export 'src/keymap/when_clause.dart';
 export 'src/dialog.dart';