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/terminal/parser_and_scroll_test.dart

@@ -40,12 +40,12 @@ class _TestViewportOffset extends ViewportOffset {
 }
 
 Widget _host(Widget child) => Directionality(
-      textDirection: TextDirection.ltr,
-      child: MediaQuery(
-        data: const MediaQueryData(),
-        child: Center(child: SizedBox(width: 400, height: 200, child: child)),
-      ),
-    );
+  textDirection: TextDirection.ltr,
+  child: MediaQuery(
+    data: const MediaQueryData(),
+    child: Center(child: SizedBox(width: 400, height: 200, child: child)),
+  ),
+);
 
 void main() {
   group('EscapeParser — explicit-param CSI cursor moves', () {