Function document_selection_presence_key 

pub fn document_selection_presence_key(
    start_block_id: &str,
    span: usize,
) -> PresenceKey

A document selection uses one transport frame: the start block’s stable id and the number of blocks it spans after that block live in the key, the existing selection payload carries the endpoint bytes. Compact enough for the live presence anchor bound (64 bytes with a UUID block id).