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/builtin/diff/src/extension.dart

@@ -24,16 +24,16 @@ class DiffExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-        TabContribution(
-          id: 'diff.view',
-          slot: Slots.workspace,
-          title: 'Diff',
-          titleKey: 'tab.title',
-          i18nNamespace: id,
-          priority: -70,
-          build: (_) => DiffView(controller: _controller),
-        ),
-      ];
+    TabContribution(
+      id: 'diff.view',
+      slot: Slots.workspace,
+      title: 'Diff',
+      titleKey: 'tab.title',
+      i18nNamespace: id,
+      priority: -70,
+      build: (_) => DiffView(controller: _controller),
+    ),
+  ];
 
   @override
   Future<void> activate(ClideExtensionContext ctx) async {