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/builtin/diff/diff_controller_test.dart

@@ -81,7 +81,13 @@ void main() {
   });
 
   test('a git.diff error surfaces on the controller', () async {
-    ipc.stub('git.diff', (_) async => IpcResponse.err(id: '', error: IpcError(code: IpcExitCode.toolError, kind: IpcErrorKind.toolError, message: 'boom')));
+    ipc.stub(
+      'git.diff',
+      (_) async => IpcResponse.err(
+        id: '',
+        error: IpcError(code: IpcExitCode.toolError, kind: IpcErrorKind.toolError, message: 'boom'),
+      ),
+    );
     await c.load();
     expect(c.error, 'boom');
   });