Status: IN
The free list head pointer is stored as the fifth field of the metadata page (page 0) in the layout `[root_page:4B][height:4B][total_keys:4B][next_free_page:4B][free_list_head:4B]`, making it durable across restarts.
Source: entries/2026/05/29/topic-intrusive-free-list-pattern.md