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/extension/src/contribution_test.dart

@@ -25,10 +25,7 @@ void main() {
     });
 
     test('StatusItemContribution pins the statusbar slot', () {
-      final s = StatusItemContribution(
-        id: 'ipc-status.indicator',
-        build: (_) => const SizedBox.shrink(),
-      );
+      final s = StatusItemContribution(id: 'ipc-status.indicator', build: (_) => const SizedBox.shrink());
       expect(s.slot, Slots.statusbar);
     });
 
@@ -42,20 +39,12 @@ void main() {
     });
 
     test('TrayItemContribution pins the tray slot', () {
-      final t = TrayItemContribution(
-        id: 't',
-        label: 'Label',
-        onSelected: () {},
-      );
+      final t = TrayItemContribution(id: 't', label: 'Label', onSelected: () {});
       expect(t.slot, Slots.tray);
     });
 
     test('ToolbarButtonContribution pins the toolbar slot', () {
-      final b = ToolbarButtonContribution(
-        id: 'save',
-        label: 'Save',
-        onPressed: () {},
-      );
+      final b = ToolbarButtonContribution(id: 'save', label: 'Save', onPressed: () {});
       expect(b.slot, Slots.toolbar);
     });
   });

test/extension/src/extension_context_test.dart

@@ -53,11 +53,13 @@ void main() {
 
     test('publish + subscribe round-trip through the message bus', () async {
       Message? received;
-      f.services.extensions.register(_SugarExt((ctx) async {
-        final first = ctx.subscribe(channel: 'greet').first;
-        ctx.publish('greet', {'hello': 'world'});
-        received = await first;
-      }));
+      f.services.extensions.register(
+        _SugarExt((ctx) async {
+          final first = ctx.subscribe(channel: 'greet').first;
+          ctx.publish('greet', {'hello': 'world'});
+          received = await first;
+        }),
+      );
       await f.services.extensions.activate('sugar.ext');
       expect(received, isNotNull);
       expect(received!.publisher, 'sugar.ext');
@@ -68,10 +70,12 @@ void main() {
     test('t + tr resolve against the extension namespace', () async {
       String? t;
       String? tr;
-      f.services.extensions.register(_SugarExt((ctx) async {
-        t = ctx.t('missing.key');
-        tr = ctx.tr('missing.key', replacers: const []);
-      }));
+      f.services.extensions.register(
+        _SugarExt((ctx) async {
+          t = ctx.t('missing.key');
+          tr = ctx.tr('missing.key', replacers: const []);
+        }),
+      );
       await f.services.extensions.activate('sugar.ext');
       // No catalog loaded for this namespace → i18n falls back, but the
       // sugar getters still execute and return a non-null string.

test/extension/src/manifest_test.dart

@@ -46,24 +46,15 @@ depends_on: [builtin.git, 42, builtin.diff]
     });
 
     test('missing id throws FormatException', () {
-      expect(
-        () => ExtensionManifest.fromYamlString('title: Floating'),
-        throwsA(isA<FormatException>()),
-      );
+      expect(() => ExtensionManifest.fromYamlString('title: Floating'), throwsA(isA<FormatException>()));
     });
 
     test('empty id throws FormatException', () {
-      expect(
-        () => ExtensionManifest.fromYamlString('id: ""'),
-        throwsA(isA<FormatException>()),
-      );
+      expect(() => ExtensionManifest.fromYamlString('id: ""'), throwsA(isA<FormatException>()));
     });
 
     test('non-map root throws FormatException', () {
-      expect(
-        () => ExtensionManifest.fromYamlString('- just a list'),
-        throwsA(isA<FormatException>()),
-      );
+      expect(() => ExtensionManifest.fromYamlString('- just a list'), throwsA(isA<FormatException>()));
     });
   });
 }