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/fake_ipc.dart

@@ -24,19 +24,12 @@ class FakeDaemonClient extends DaemonClient {
   }
 
   @override
-  Future<IpcResponse> request(
-    String cmd, {
-    Map<String, Object?> args = const {},
-  }) async {
+  Future<IpcResponse> request(String cmd, {Map<String, Object?> args = const {}}) async {
     final stub = _stubs[cmd];
     if (stub != null) return stub(args);
     return IpcResponse.err(
       id: '',
-      error: IpcError(
-        code: IpcExitCode.notFound,
-        kind: IpcErrorKind.notFound,
-        message: 'no stub for $cmd',
-      ),
+      error: IpcError(code: IpcExitCode.notFound, kind: IpcErrorKind.notFound, message: 'no stub for $cmd'),
     );
   }
 
@@ -46,10 +39,7 @@ class FakeDaemonClient extends DaemonClient {
     notifyListeners();
   }
 
-  void stub(
-    String cmd,
-    Future<IpcResponse> Function(Map<String, Object?>) handler,
-  ) {
+  void stub(String cmd, Future<IpcResponse> Function(Map<String, Object?>) handler) {
     _stubs[cmd] = handler;
   }
 }

test/helpers/golden_harness.dart

@@ -9,11 +9,7 @@ import 'package:alchemist/alchemist.dart';
 AlchemistConfig clideGoldenConfig() {
   return const AlchemistConfig(
     theme: null, // we're not using Material ThemeData
-    platformGoldensConfig: PlatformGoldensConfig(
-      enabled: true,
-    ),
-    ciGoldensConfig: CiGoldensConfig(
-      enabled: false,
-    ),
+    platformGoldensConfig: PlatformGoldensConfig(enabled: true),
+    ciGoldensConfig: CiGoldensConfig(enabled: false),
   );
 }

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));
 }

test/helpers/widget_harness.dart

@@ -21,14 +21,7 @@ Widget harness(KernelFixture fixture, Widget child) {
         controller: fixture.services.theme,
         child: MediaQuery(
           data: const MediaQueryData(),
-          child: Overlay(
-            initialEntries: [
-              OverlayEntry(
-                canSizeOverlay: true,
-                builder: (_) => child,
-              ),
-            ],
-          ),
+          child: Overlay(initialEntries: [OverlayEntry(canSizeOverlay: true, builder: (_) => child)]),
         ),
       ),
     ),
@@ -45,12 +38,7 @@ Widget harness(KernelFixture fixture, Widget child) {
 /// with [child] (the trigger) placed at [alignment]. Set
 /// `tester.view.physicalSize = size` to match (the default 800×600 matches the
 /// default test view, so no setup is needed unless you change [size]).
-Widget anchoredHarness(
-  KernelFixture fixture,
-  Widget child, {
-  Size size = const Size(800, 600),
-  Alignment alignment = Alignment.topLeft,
-}) {
+Widget anchoredHarness(KernelFixture fixture, Widget child, {Size size = const Size(800, 600), Alignment alignment = Alignment.topLeft}) {
   return Directionality(
     textDirection: TextDirection.ltr,
     child: ClideKernel(
@@ -61,7 +49,9 @@ Widget anchoredHarness(
           data: MediaQueryData(size: size),
           child: Overlay(
             initialEntries: [
-              OverlayEntry(builder: (_) => Align(alignment: alignment, child: child)),
+              OverlayEntry(
+                builder: (_) => Align(alignment: alignment, child: child),
+              ),
             ],
           ),
         ),