- Correctness: _sync_overlay now first-wins on a guid (a `seen` set) so a duplicate HU entry
sharing a guid can't overwrite the applied Hungarian metadata or double-count `overlaid`.
- Test-coverage: cover the earlier dup-guid fix (both editions of a shared-guid pair get *_hu,
counted once) and the guid-keep guard (_apply_item preserves a prior guid when the listing
carries none, still updates on a real one).
Full backend suite 276 passed / 2 skipped.