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/facade.dart

@@ -147,13 +147,7 @@ class KernelServices {
     final settings = SettingsStore(appDir: appDir);
     await settings.load();
 
-    final i18n = I18n(
-      loader: i18nLoader,
-      log: log,
-      defaultLocale: defaultLocale,
-      initialLocale: initialLocale,
-      availableLocales: availableLocales,
-    );
+    final i18n = I18n(loader: i18nLoader, log: log, defaultLocale: defaultLocale, initialLocale: initialLocale, availableLocales: availableLocales);
     for (final ns in preloadNamespaces) {
       await i18n.ensureNamespaceLoaded(ns);
     }
@@ -193,7 +187,8 @@ class KernelServices {
       onProjectOpen: onProjectOpen,
       onValidateProject: onValidateProject,
     );
-    final ipc = isolateClient ??
+    final ipc =
+        isolateClient ??
         (daemonClientFactory != null
             ? daemonClientFactory(log, events, arrangement, panels)
             : DaemonClient(
@@ -309,11 +304,7 @@ class KernelServices {
 }
 
 class ClideKernel extends InheritedWidget {
-  const ClideKernel({
-    super.key,
-    required this.services,
-    required super.child,
-  });
+  const ClideKernel({super.key, required this.services, required super.child});
 
   final KernelServices services;