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>

test/helpers/kernel_fixture.dart

@@ -34,7 +34,7 @@ class KernelFixture {
       preloadNamespaces: preloadNamespaces ?? catalogs.keys.toList(),
       defaultLocale: defaultLocale,
       initialLocale: initialLocale,
-      daemonClientFactory: (log, events, _, __) {
+      daemonClientFactory: (log, events, _, _) {
         fake = FakeDaemonClient(log: log, events: events);
         return fake!;
       },
@@ -45,11 +45,7 @@ class KernelFixture {
       // ~10 minutes (T-280); `existsSync` opens no native port, so it's safe.
       onValidateProject: onValidateProject ?? _walkForGitRoot,
     );
-    return KernelFixture._(
-      services: services,
-      ipc: fake!,
-      tempDir: tempDir,
-    );
+    return KernelFixture._(services: services, ipc: fake!, tempDir: tempDir);
   }
 
   Future<void> dispose() async {
@@ -100,10 +96,5 @@ ThemeDefinition _miniTheme() {
     'error': Color(0xFFF06C6F),
     'info': Color(0xFF00A3D2),
   };
-  return const ThemeDefinition(
-    name: 'test',
-    displayName: 'Test',
-    dark: true,
-    palette: Palette(palette),
-  );
+  return const ThemeDefinition(name: 'test', displayName: 'Test', dark: true, palette: Palette(palette));
 }