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/lua/src/adapter.dart

@@ -4,12 +4,7 @@ import 'package:clide/extension/extension.dart';
 /// and a handle to the Lua state; contributions are proxied to
 /// callbacks registered by `clide.contribute(...)` from the Lua side.
 class LuaExtension extends ClideExtension {
-  LuaExtension({
-    required this.id,
-    required this.title,
-    required this.version,
-    this.dependsOn = const [],
-  });
+  LuaExtension({required this.id, required this.title, required this.version, this.dependsOn = const []});
 
   @override
   final String id;