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/kernel/src/clipboard.dart

@@ -14,10 +14,7 @@ class ClideClipboard {
   final int historyLimit;
   final Map<Type, List<Object>> _history = {};
 
-  Future<void> write<T extends Object>(
-    T value, {
-    String Function(T)? toPlain,
-  }) async {
+  Future<void> write<T extends Object>(T value, {String Function(T)? toPlain}) async {
     final bucket = _history.putIfAbsent(T, () => <Object>[]);
     bucket.insert(0, value);
     if (bucket.length > historyLimit) bucket.removeLast();