Final review follow-ups: the folder guard compares against the home folder,
so a cache hit now also needs Path.home() unchanged. TESTING.md says the
configured-path rule runs on the cache's result and that a non-UTF-8
sidecar is skipped.
CI: 'Rebuild all is explicit...' (webkit) polled the job for the default
5 s while it regenerated 24 previews; the failure screenshot shows 12 of 24
done at the deadline on the busy runner (0.17 s locally). The poll now
allows 30 s for that real work.