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_tooltip_test.dart

@@ -112,12 +112,7 @@ void main() {
 
       // The Positioned ancestor of the tooltip uses `bottom:` (above-mode),
       // not `top:`, when there isn't enough room below.
-      final positioned = tester.widget<Positioned>(
-        find.ancestor(
-          of: find.text('above'),
-          matching: find.byType(Positioned),
-        ),
-      );
+      final positioned = tester.widget<Positioned>(find.ancestor(of: find.text('above'), matching: find.byType(Positioned)));
       expect(positioned.bottom, isNotNull);
       expect(positioned.top, isNull);
     });