Function make_unified_diff 

pub fn make_unified_diff(path: &str, old: &str, new: &str) -> String

Build a unified diff between old and new keyed at path.

Returns an empty string when the inputs are byte-identical so callers can skip the “no changes” header. The output uses git-style --- a/... / +++ b/... headers and three lines of context — matching the format the TUI’s diff_render::render_diff already understands.