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/files.dart

@@ -16,10 +16,7 @@ class FilesDropped extends ClideEvent {
   @override
   String get kind => 'dropped';
   @override
-  Map<String, Object?> payload() => {
-        'paths': paths,
-        'slot': slot.value,
-      };
+  Map<String, Object?> payload() => {'paths': paths, 'slot': slot.value};
 }
 
 /// Tier-0 stub for file pickers and drop targets.
@@ -33,17 +30,11 @@ class FileServices {
   FileServices(this._events);
   final DaemonBus _events;
 
-  Future<List<String>> pickOpen({
-    List<String> extensions = const [],
-    bool multiple = false,
-  }) async {
+  Future<List<String>> pickOpen({List<String> extensions = const [], bool multiple = false}) async {
     throw UnimplementedError('pickOpen — wired in a later tier');
   }
 
-  Future<String?> pickSave({
-    String? defaultName,
-    List<String> extensions = const [],
-  }) async {
+  Future<String?> pickSave({String? defaultName, List<String> extensions = const []}) async {
     throw UnimplementedError('pickSave — wired in a later tier');
   }