Function hunk_fold_identity 

pub fn hunk_fold_identity(start_line: u32, end_line: u32) -> u64

Compute the stable identity hash for a single hunk fold.

Namespaced with "diff:hunk" so the hash doesn’t collide with a primary provider’s hash for the same (start_line, end_line). Two publishes that produce a hunk at the same span hash identically, so closed-state carries over.