The spill manager is process-global and never dropped, so the Unix scratch file was only
removed by the next launch's reap. Unlink it immediately after opening and read/write
through the one handle with pread/pwrite (readers can no longer reopen it by path).
Windows keeps delete-on-close and per-thread readers. The abrupt-exit test now asserts
that no file remains on every platform; docs/scratch-disks.md updated.