Function compute_diff 

pub fn compute_diff(old: &XcStringsFile, new: &XcStringsFile) -> DiffReport

Compare two versions of an XcStringsFile and return the differences. old is typically the cached version, new is the freshly-read version from disk.

Compares structural changes (added/removed keys) and source language text changes. Translation changes in non-source locales are NOT detected — this is intentional, as the primary use case is detecting source content changes that require re-translation.