Adds mirror-registry as a featured project (order 26, beside The Exploded
Cluster) covering the zot deployment, the bootstrap and auth circular
dependencies, and the retention/revalidation behaviours that corrected the
first published write-up. Adds the cover: field to exploded-cluster (it had
none, so the card rendered imageless) and stages public/covers/image.md with
the generation prompts and processing steps for both covers.
jwright