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/widgets/quick_open_overlay_test.dart

@@ -19,11 +19,15 @@ void main() {
   setUp(() async {
     f = await KernelFixture.create();
     f.ipc.stub(
-        'files.walk',
-        (_) async => IpcResponse.ok(id: '1', data: const {
-              'files': ['lib/main.dart', 'lib/app.dart', 'README.md'],
-              'truncated': false,
-            }));
+      'files.walk',
+      (_) async => IpcResponse.ok(
+        id: '1',
+        data: const {
+          'files': ['lib/main.dart', 'lib/app.dart', 'README.md'],
+          'truncated': false,
+        },
+      ),
+    );
     f.ipc.stub('editor.open', (args) async => IpcResponse.ok(id: '1', data: {'path': args['path']}));
   });
   tearDown(() => f.dispose());
@@ -102,11 +106,15 @@ void main() {
 
   testWidgets('truncated walk surfaces the limited-results hint', (tester) async {
     f.ipc.stub(
-        'files.walk',
-        (_) async => IpcResponse.ok(id: '1', data: const {
-              'files': ['lib/main.dart'],
-              'truncated': true,
-            }));
+      'files.walk',
+      (_) async => IpcResponse.ok(
+        id: '1',
+        data: const {
+          'files': ['lib/main.dart'],
+          'truncated': true,
+        },
+      ),
+    );
     await tester.pumpWidget(harness(f, const QuickOpenOverlay()));
     f.services.quickOpen.open();
     await pumpAsync(tester);
@@ -151,11 +159,12 @@ void main() {
 
   testWidgets('files.walk failure leaves the list empty (no crash)', (tester) async {
     f.ipc.stub(
-        'files.walk',
-        (_) async => IpcResponse.err(
-              id: '1',
-              error: IpcError(code: IpcExitCode.toolError, kind: IpcErrorKind.toolError, message: 'boom'),
-            ));
+      'files.walk',
+      (_) async => IpcResponse.err(
+        id: '1',
+        error: IpcError(code: IpcExitCode.toolError, kind: IpcErrorKind.toolError, message: 'boom'),
+      ),
+    );
     await tester.pumpWidget(harness(f, const QuickOpenOverlay()));
     f.services.quickOpen.open();
     await pumpAsync(tester);