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/tool_check.dart

@@ -12,11 +12,7 @@ class ToolCheck extends ChangeNotifier {
 
   bool get allOk => pqlOk && tmuxOk && gitOk;
 
-  List<String> get errors => [
-        if (!pqlOk) 'pql not found',
-        if (!tmuxOk) 'tmux not found',
-        if (!gitOk) 'git not found',
-      ];
+  List<String> get errors => [if (!pqlOk) 'pql not found', if (!tmuxOk) 'tmux not found', if (!gitOk) 'git not found'];
 
   /// Workspace root, set by the app at boot. Falls back to cwd.
   static String? workspaceRoot;