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/daemon/status_command_test.dart

@@ -13,13 +13,7 @@ void main() {
 
   test('status resolves and returns the assembled snapshot with exit 0', () async {
     final d = DaemonDispatcher();
-    registerStatusCommand(
-        d,
-        () async => {
-              'workspace': '/repo',
-              'focusedFile': null,
-              'panes': const [],
-            });
+    registerStatusCommand(d, () async => {'workspace': '/repo', 'focusedFile': null, 'panes': const []});
     final r = await d.dispatch(req());
     expect(r.ok, isTrue);
     expect(r.data['workspace'], '/repo');