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/terminal/buffer/buffer_test.dart

@@ -14,14 +14,7 @@ import 'package:clide/src/terminal/src/core/state.dart';
 import 'package:test/test.dart';
 
 class _State implements TerminalState {
-  _State({
-    this.viewWidth = 10,
-    this.viewHeight = 5,
-    this.autoWrapMode = true,
-    this.lineFeedMode = false,
-    this.originMode = false,
-    this.reflowEnabled = false,
-  });
+  _State({this.viewWidth = 10, this.viewHeight = 5, this.autoWrapMode = true, this.lineFeedMode = false, this.originMode = false, this.reflowEnabled = false});
 
   @override
   int viewWidth;
@@ -83,12 +76,7 @@ Buffer _newBuffer({
     originMode: originMode,
     reflowEnabled: reflowEnabled,
   );
-  return Buffer(
-    state,
-    maxLines: maxLines,
-    isAltBuffer: isAltBuffer,
-    wordSeparators: wordSeparators,
-  );
+  return Buffer(state, maxLines: maxLines, isAltBuffer: isAltBuffer, wordSeparators: wordSeparators);
 }
 
 void main() {