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/default_layout/widget_test.dart

@@ -40,10 +40,7 @@ void main() {
 
     test('declares a layout preset contribution', () {
       final ext = DefaultLayoutExtension();
-      expect(
-        ext.contributions.whereType<LayoutPresetContribution>(),
-        hasLength(1),
-      );
+      expect(ext.contributions.whereType<LayoutPresetContribution>(), hasLength(1));
     });
 
     test('all commands return not-activated errors before activate', () async {