Route skill backup selection and verification through the shared recovery owner with the installer resource scope. Reuse verified snapshots without copying and remove duplicate selection and EEXIST handling.
Cover failed closes at first backup, recovery backup, and post-copy verification; block new descriptors while cleanup is pending and retain the exact owner through disposal retries. Related: #4592.
Validation: 158 focused tests and 511 adjacent setup tests passed; check:fast, docs:check and formatting passed. Close failures were injected on real file handles on macOS; native Windows/Linux faults were not reproduced.