DatumFieldSpec<E, V> class

A full runtime descriptor for one serialized field of entity E with Dart value type V.

Declared once per field as static final (the codec/getter closures rule out const), the same declare-once pattern as MemoizedRelations:

abstract final class TaskFields {
  static final title = DatumFieldSpec<Task, String>('title', getter: (t) => t.title);
  static final priority = DatumFieldSpec<Task, int>('priority',
      getter: (t) => t.priority, defaultValue: 0, renamedFrom: 'prio');
}

// Everything DatumQueryField offers still works:
final query = DatumQueryBuilder<Task>().whereField(TaskFields.priority, isGreaterThan: 2).build();