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>

lib/builtin/diff/src/diff_controller.dart

@@ -50,18 +50,12 @@ class DiffController extends ChangeNotifier {
   }
 
   /// Load diffs. Optionally filter to [paths] and toggle [staged].
-  Future<void> load({
-    bool staged = false,
-    List<String> paths = const [],
-  }) async {
+  Future<void> load({bool staged = false, List<String> paths = const []}) async {
     _staged = staged;
     _loading = true;
     notifyListeners();
 
-    final r = await ipc.request('git.diff', args: {
-      'staged': staged,
-      if (paths.isNotEmpty) 'paths': paths,
-    });
+    final r = await ipc.request('git.diff', args: {'staged': staged, if (paths.isNotEmpty) 'paths': paths});
 
     _loading = false;
     if (!r.ok) {