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/builtin/editor/src/editor_controller.dart

@@ -99,12 +99,7 @@ class EditorController extends ChangeNotifier {
     if (raw is! List) return;
     _buffers = [
       for (final b in raw)
-        if (b is Map)
-          (
-            id: b['id']! as String,
-            path: b['path']! as String,
-            dirty: (b['dirty'] as bool?) ?? false,
-          ),
+        if (b is Map) (id: b['id']! as String, path: b['path']! as String, dirty: (b['dirty'] as bool?) ?? false),
     ];
     notifyListeners();
   }
@@ -143,10 +138,7 @@ class EditorController extends ChangeNotifier {
   }
 
   /// Called by the widget on every local text edit.
-  void pushLocalEdit({
-    required String newContent,
-    required Selection newSelection,
-  }) {
+  void pushLocalEdit({required String newContent, required Selection newSelection}) {
     final id = _activeId;
     if (id == null) return;
 
@@ -162,11 +154,7 @@ class EditorController extends ChangeNotifier {
     // large buffers, so event broadcasts stay small.
     _pendingLocalEdits++;
     _suppressNextRemoteEdit = true;
-    ipc.request('editor.set-content', args: {
-      'id': id,
-      'text': newContent,
-      'selection': newSelection.toJson(),
-    }).whenComplete(() => _pendingLocalEdits--);
+    ipc.request('editor.set-content', args: {'id': id, 'text': newContent, 'selection': newSelection.toJson()}).whenComplete(() => _pendingLocalEdits--);
   }
 
   Future<void> save() async {