chore: adopt Dart 3.9 toolchain — honest floor + tall-style reformat (T-353)

Raise the declared minimums in pubspec.yaml to what our deps already
require: Flutter >=3.35.0 / Dart >=3.9.0 (was 3.19.0 / 3.5.0). alchemist
0.12 needs Flutter 3.32; Dart 3.9 first ships in Flutter 3.35, so 3.35 is
the binding floor. Pin the exact build toolchain in .fvmrc (Flutter
3.44.1).

Moving to the Dart 3.9 language level switches `dart format` to the new
"tall" style and enables two new lints. This commit is the resulting
mechanical churn, isolated from any behaviour change:
  - whole-tree `dart format` reformat (tall style)
  - `dart fix` for unnecessary_underscores + use_null_aware_elements

No runtime behaviour change; `make test` green.

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

integration_test/extension_lifecycle_test.dart

@@ -25,22 +25,13 @@ void main() {
       tester.view.resetDevicePixelRatio();
     });
 
-    final themes = [
-      await const ThemeLoader().fromAsset(
-        rootBundle,
-        'lib/kernel/src/theme/themes/summer-night.yaml',
-      ),
-    ];
+    final themes = [await const ThemeLoader().fromAsset(rootBundle, 'lib/kernel/src/theme/themes/summer-night.yaml')];
     final services = await KernelServices.boot(
       appDir: await Directory.systemTemp.createTemp('clide_lc_'),
       bundledThemes: themes,
       i18nLoader: AssetCatalogLoader(bundle: rootBundle),
-      preloadNamespaces: const [
-        'builtin.welcome',
-        'builtin.ipc-status',
-        'builtin.default-layout',
-      ],
-      daemonClientFactory: (log, events, _, __) => FakeDaemonClient(log: log, events: events),
+      preloadNamespaces: const ['builtin.welcome', 'builtin.ipc-status', 'builtin.default-layout'],
+      daemonClientFactory: (log, events, _, _) => FakeDaemonClient(log: log, events: events),
       autoStartDaemonClient: false,
     );
     services.extensions