Function unchanged_fold_identity 

pub fn unchanged_fold_identity(
    slot: usize,
    start_line: u32,
    end_line: u32,
) -> u64

Compute the stable identity hash for a single unchanged fold. Namespaced with "diff:unchanged" + the slot so it never collides with a hunk fold ("diff:hunk") or the same span on the other side; span-keyed so closed-state (a user zo) survives a republish that reproduces the same gap.