rename() spread the entire fetched entity into brain.update(), forwarding a
vector field that failed dimension validation when the entity was fetched
without vectors. A rename is a path/metadata change: the update is now
metadata-only — no vector forwarded, no re-embedding, no vector-index touch.
Regression suite ported from the 7.x line (verbatim consumer repro,
cross-directory move, EEXIST). The companion rollback fix from 7.31.7 was
already present on this branch via the pre-RC1 sweep.
