Function register_handle_fields 

pub fn register_handle_fields(template: ClassTemplateId, slots: Vec<FieldSlot>)

Registers (idempotently — a later call replaces, never merges, an earlier one for the same template) the ordered list of template’s own non-static fields that are themselves class handles, consumed by release’s recursive free walk. Task 3 stub (this task’s own tests call it directly with a hand-built list); Task 5 wires in the real ObjectLayout-derived list once that type exists.