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/widgets/src/clide_marquee_test.dart

@@ -9,16 +9,13 @@ import 'package:flutter_test/flutter_test.dart';
 import '../../helpers/widget_harness.dart';
 
 Widget _boxed(double width, Widget child) => Directionality(
-      textDirection: TextDirection.ltr,
-      child: Center(
-        child: SizedBox(width: width, height: 16, child: child),
-      ),
-    );
+  textDirection: TextDirection.ltr,
+  child: Center(
+    child: SizedBox(width: width, height: 16, child: child),
+  ),
+);
 
-Widget _reducedMotion(double width, Widget child) => MediaQuery(
-      data: const MediaQueryData(disableAnimations: true),
-      child: _boxed(width, child),
-    );
+Widget _reducedMotion(double width, Widget child) => MediaQuery(data: const MediaQueryData(disableAnimations: true), child: _boxed(width, child));
 
 void main() {
   testWidgets('shows the child statically when it fits', (tester) async {