Retain the staged file descriptor through atomic publication, readback and
recovery. Refuse detected replacements or edits and restore through that
owned descriptor instead of reopening a pathname. First-run readback failure
now retains the file for manual recovery because pathname deletion cannot
prove exact ownership. Keep both descriptor owners when nested closes fail;
opaque close rejections do not transfer ownership.
Validation: 192 focused checks in 9 files, check:fast, formatting and docs:check
passed. Three replacement regressions fail on the baseline while six existing
recovery controls pass. Six completed setup tasks preserve equivalent config
and backups. Two read-only source reviews completed on macOS arm64 / Node24.
Native Windows/Linux and real concurrent editor behavior remain unverified.
State checks do not exclude an in-place writer after the last check.
Closes#4900
morluto
7e4eed9dd4