add Vim mode service, mode commands, and status indicator

T-207, first foundation piece of the Vim layer (T-65 epic). A
VimModeService (ChangeNotifier) owns the normal/insert/visual mode and
mirrors it into the keymap as mutually-exclusive vim.normal/vim.insert/
vim.visual scope flags. Those flags are the public mode interface: the
editor (T-206) will read them to decide insert-vs-command, and vim.yaml
(T-65) guards bindings with `when: vim.*`. Nothing reaches across the
builtin boundary into the service object.

The layer is gated on the active preset — the builtin.vim extension
ties VimModeService.enabled to app.keymap.preset and re-checks on every
keymap reload, so i/v/Esc never hijack input under non-Vim presets. Mode
commands (vim.mode.{normal,insert,visual}) carry no default binding for
the same reason; only vim.yaml binds keys to them. A status-bar item
shows `-- NORMAL --` etc. while enabled.

Exposes KeymapService on the extension context so the layer can publish
scope flags.

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

lib/builtin/vim/src/vim_mode_indicator.dart

@@ -0,0 +1,33 @@
+import 'package:clide/builtin/vim/src/vim_mode_service.dart';
+import 'package:clide/kernel/kernel.dart';
+import 'package:clide/widgets/widgets.dart';
+import 'package:flutter/widgets.dart';
+
+/// Status-bar item showing the current Vim mode (`-- NORMAL --`). Renders
+/// nothing while the Vim layer is disabled, so it's invisible under
+/// non-Vim presets (T-207).
+class VimModeIndicator extends StatelessWidget {
+  const VimModeIndicator({super.key, required this.service});
+
+  final VimModeService service;
+
+  @override
+  Widget build(BuildContext context) {
+    return ListenableBuilder(
+      listenable: service,
+      builder: (context, _) {
+        if (!service.enabled) return const SizedBox.shrink();
+        final tokens = ClideTheme.of(context).surface;
+        return Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 8),
+          child: ClideText(
+            '-- ${service.mode.label} --',
+            fontFamily: clideMonoFamily,
+            fontSize: clideFontCaption,
+            color: tokens.statusBarForeground,
+          ),
+        );
+      },
+    );
+  }
+}