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/focus.dart

@@ -18,11 +18,7 @@ class FocusTracker extends ChangeNotifier {
   /// of the three-column layout (sidebar → workspace → context); the
   /// statusbar / toolbar aren't included because they don't host
   /// keyboard-active content.
-  static const List<SlotId> traversalOrder = [
-    Slots.sidebar,
-    Slots.workspace,
-    Slots.contextPanel,
-  ];
+  static const List<SlotId> traversalOrder = [Slots.sidebar, Slots.workspace, Slots.contextPanel];
 
   SlotId? get activeSlot => _slot;
   String? get activeContributionId => _contributionId;