fix: move ValidationResult.invalid constructor before fields (sort_constructors_first)

lib/domain/services/unit_tree_service.dart

@@ -5,18 +5,18 @@ import 'package:uto_manager/domain/entities/unit_node.dart';
 
 /// Result of a doctrinal validation check.
 class ValidationResult {
+  /// Creates a [ValidationResult] with explicit [isValid] and optional [reason].
   const ValidationResult({required this.isValid, this.reason});
 
+  /// Creates a failing validation result with a [reason].
+  ValidationResult.invalid(this.reason) : isValid = false;
+
   /// Whether the operation is doctrinally valid.
   final bool isValid;
 
   /// Human-readable reason if [isValid] is false.
   final String? reason;
 
-  // ignore: sort_constructors_first
-  /// Creates a failing validation result with a [reason].
-  ValidationResult.invalid(this.reason) : isValid = false;
-
   /// A passing validation result.
   static const valid = ValidationResult(isValid: true);
 }