Preserve unlinked overlay handles when deferring copy-up

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.

Start by reading the deferred overlay copy-up work in #1318 and trace how lower-backed handles behave after unlink. Exercise the listed cases—replaced paths, detached staging, concurrent handles, and upper-handle lifetime—and consider the work complete when existing handles update without recreating the pathname while new opens report it missing.

Written by the indexing model from the issue text.