Introduce a dedicated `/space/$namespace/$slug/compare` page that compares
two published skill versions GitHub-style: left file list + right unified
diff. Backend exposes `GET /versions/compare` returning structured diff
(computed via java-diff-utils) with per-file hunks, binary placeholder,
and truncation flags. Frontend uses two version selectors scoped to
PUBLISHED versions, a file search box, active-file highlighting, and
whitespace-preserving unified view. E2E covers the publish + rerelease
+ approve round trip; controller/domain tests cover happy path and
same-version rejection.
