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/builtin/editor/vim_editor_test.dart

@@ -22,26 +22,29 @@ void main() {
 
   void stubOneBuffer(String content) {
     f.ipc.stub(
-        'editor.list',
-        (_) async => _ok({
-              'buffers': [
-                {'id': 'b_1', 'path': 'lib/a.dart', 'dirty': false}
-              ]
-            }));
+      'editor.list',
+      (_) async => _ok({
+        'buffers': [
+          {'id': 'b_1', 'path': 'lib/a.dart', 'dirty': false},
+        ],
+      }),
+    );
     f.ipc.stub(
-        'editor.active',
-        (_) async => _ok({
-              'active': {'id': 'b_1'}
-            }));
+      'editor.active',
+      (_) async => _ok({
+        'active': {'id': 'b_1'},
+      }),
+    );
     f.ipc.stub(
-        'editor.read',
-        (_) async => _ok({
-              'id': 'b_1',
-              'path': 'lib/a.dart',
-              'content': content,
-              'selection': {'start': 0, 'end': 0},
-              'dirty': false,
-            }));
+      'editor.read',
+      (_) async => _ok({
+        'id': 'b_1',
+        'path': 'lib/a.dart',
+        'content': content,
+        'selection': {'start': 0, 'end': 0},
+        'dirty': false,
+      }),
+    );
   }
 
   Future<void> pumpEditor(WidgetTester tester) async {