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>

lib/kernel/src/toolchain.dart

@@ -44,11 +44,7 @@ class Toolchain extends ChangeNotifier implements ToolchainView {
   bool get allOk => _resolved && missing.isEmpty;
 
   @override
-  List<String> get missing => [
-        if (_git == null) 'git',
-        if (_pql == null) 'pql',
-        if (_tmux == null) 'tmux',
-      ];
+  List<String> get missing => [if (_git == null) 'git', if (_pql == null) 'pql', if (_tmux == null) 'tmux'];
 
   /// Returns a Future that completes when resolution finishes.
   Future<void> waitForResolution() {