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/secrets.dart

@@ -6,25 +6,15 @@
 class SecretsVault {
   final Map<String, String> _memory = {};
 
-  Future<void> write({
-    required String extensionId,
-    required String key,
-    required String value,
-  }) async {
+  Future<void> write({required String extensionId, required String key, required String value}) async {
     _memory['$extensionId/$key'] = value;
   }
 
-  Future<String?> read({
-    required String extensionId,
-    required String key,
-  }) async {
+  Future<String?> read({required String extensionId, required String key}) async {
     return _memory['$extensionId/$key'];
   }
 
-  Future<void> delete({
-    required String extensionId,
-    required String key,
-  }) async {
+  Future<void> delete({required String extensionId, required String key}) async {
     _memory.remove('$extensionId/$key');
   }