Function prune_dangling_stack_members 

fn prune_dangling_stack_members(
    node: &mut LayoutNode,
    live_block_ids: &HashSet<String>,
) -> usize

Recursively drop dead entries from every leaf’s block_stack. The active member (data.block_id) is never removed here — by the time this runs, the caller’s whole-leaf pruning loop has already guaranteed it’s live. Returns the number of stack entries removed.