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_surface_test.dart

@@ -13,9 +13,7 @@ void main() {
     tearDown(() async => f.dispose());
 
     testWidgets('default background comes from panelBackground token', (tester) async {
-      await tester.pumpWidget(
-        harness(f, const ClideSurface(child: Text('x'))),
-      );
+      await tester.pumpWidget(harness(f, const ClideSurface(child: Text('x'))));
       final container = tester.widget<Container>(find.byType(Container));
       final decoration = container.decoration as BoxDecoration;
       expect(decoration.color, f.services.theme.current.surface.panelBackground);
@@ -23,24 +21,14 @@ void main() {
 
     testWidgets('explicit color overrides the token default', (tester) async {
       const custom = Color(0xFF123456);
-      await tester.pumpWidget(
-        harness(f, const ClideSurface(color: custom, child: Text('x'))),
-      );
+      await tester.pumpWidget(harness(f, const ClideSurface(color: custom, child: Text('x'))));
       final container = tester.widget<Container>(find.byType(Container));
       final decoration = container.decoration as BoxDecoration;
       expect(decoration.color, custom);
     });
 
     testWidgets('border when provided renders a BoxBorder', (tester) async {
-      await tester.pumpWidget(
-        harness(
-          f,
-          ClideSurface(
-            border: f.services.theme.current.surface.modalSurfaceBorder,
-            child: const Text('x'),
-          ),
-        ),
-      );
+      await tester.pumpWidget(harness(f, ClideSurface(border: f.services.theme.current.surface.modalSurfaceBorder, child: const Text('x'))));
       final container = tester.widget<Container>(find.byType(Container));
       final decoration = container.decoration as BoxDecoration;
       expect(decoration.border, isNotNull);