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>

test/a11y/i18n_coverage_test.dart

@@ -16,29 +16,10 @@ void main() {
   /// (namespace, key) pairs referenced by Tier-0 built-ins. Extend when
   /// new keys land.
   const referenced = <String, List<String>>{
-    'builtin.welcome': [
-      'title',
-      'subtitle',
-      'open-project',
-      'open-project.hint',
-      'tab.title',
-    ],
-    'builtin.ipc-status': [
-      'connected',
-      'connected.hint',
-      'disconnected',
-      'disconnected.hint',
-    ],
-    'builtin.theme-picker': [
-      'modal.title',
-      'modal.cancel',
-      'modal.cancel.hint',
-      'row.select.hint',
-    ],
-    'builtin.default-layout': [
-      'command.reset',
-      'preset.classic',
-    ],
+    'builtin.welcome': ['title', 'subtitle', 'open-project', 'open-project.hint', 'tab.title'],
+    'builtin.ipc-status': ['connected', 'connected.hint', 'disconnected', 'disconnected.hint'],
+    'builtin.theme-picker': ['modal.title', 'modal.cancel', 'modal.cancel.hint', 'row.select.hint'],
+    'builtin.default-layout': ['command.reset', 'preset.classic'],
   };
 
   group('i18n coverage (Tier 0)', () {
@@ -47,17 +28,9 @@ void main() {
       test('$ns catalog contains every referenced key', () async {
         final loader = AssetCatalogLoader(bundle: rootBundle);
         final catalog = await loader.load(ns, const Locale('en', 'US'));
-        expect(
-          catalog,
-          isNotEmpty,
-          reason: 'catalog for "$ns" failed to load (asset path wrong?)',
-        );
+        expect(catalog, isNotEmpty, reason: 'catalog for "$ns" failed to load (asset path wrong?)');
         for (final key in entry.value) {
-          expect(
-            catalog.containsKey(key),
-            isTrue,
-            reason: 'namespace "$ns" catalog is missing key "$key"',
-          );
+          expect(catalog.containsKey(key), isTrue, reason: 'namespace "$ns" catalog is missing key "$key"');
         }
       });
     }

test/a11y/keyboard_traversal_test.dart

@@ -34,9 +34,7 @@ void main() {
     });
 
     testWidgets('interactive widgets expose tap actions to a11y', (tester) async {
-      await tester.pumpWidget(
-        harness(f, ClideButton(label: 'Save', onPressed: () {})),
-      );
+      await tester.pumpWidget(harness(f, ClideButton(label: 'Save', onPressed: () {})));
       final handle = tester.ensureSemantics();
       final data = tester.getSemantics(find.byType(ClideButton)).getSemanticsData();
       expect(data.hasAction(SemanticsAction.tap), isTrue);

test/a11y/semantic_coverage_test.dart

@@ -17,12 +17,7 @@ import 'package:flutter_test/flutter_test.dart';
 /// catalog" gate.
 void main() {
   group('Tier-0 built-in extensions — contract-level coverage', () {
-    final extensions = <ClideExtension>[
-      DefaultLayoutExtension(),
-      WelcomeExtension(),
-      IpcStatusExtension(),
-      ThemePickerExtension(),
-    ];
+    final extensions = <ClideExtension>[DefaultLayoutExtension(), WelcomeExtension(), IpcStatusExtension(), ThemePickerExtension()];
 
     for (final ext in extensions) {
       group(ext.id, () {