Any future deferred overlay copy-up must preserve unlink semantics: writing through an already-open lower-backed handle after unlink must update that handle without recreating the removed pathname, while new opens must continue to report the path as missing. Cover replaced paths, detached staging, concurrent handles, and upper-handle lifetime. Deferred from #1318 pending design review with the original fs author.
Any future deferred overlay copy-up must preserve unlink semantics: writing through an already-open lower-backed handle after unlink must update that handle without recreating the removed pathname, while new opens must continue to report the path as missing. Cover replaced paths, detached staging, concurrent handles, and upper-handle lifetime. Deferred from #1318 pending design review with the original fs author.