found while generating the reading-list preview.
the avatar preparer used the operating-system temp directory, then renamed into the project cache. those can be different filesystems on spindle, producing EXDEV. create the temporary directory beside the final cached image instead, preserving the atomic rename and cleanup.
five regression tests cover fresh writes, cache reuse, offline fallback, conversion cleanup, and an actual two-filesystem case. the unpatched code fails exactly that final test; the fix passes all five. full local suite: 21 passing, plus lint, astro check and diff check.
this fixes the site plumbing; no changes to alice's contribution.