#4016 Docs: doctor datastores reference — unreadable data version check and repair (swamp-club#3955)

swamp-club#3955 adds to swamp doctor datastores on the v2 filesystem datastore:

• A new unreadable_versions health check. It fails the scan when a published data version has missing, empty or unparseable metadata.yaml, missing raw, raw shorter than its recorded size, a catalog row with no version directory, or an unreadable latest marker. JSON output gains an unreadableVersionsFinding field.
• A new --repair stage. It previews, then with -y rolls each affected record back to its last readable version (removing unreadable versions and fixing the latest marker and catalog flags), or removes the record when no readable version is left.
• A read-time warning: data get, data query and CEL data reads warn once per unreadable version and point at swamp doctor datastores --repair.

Page to update

content/manual/reference/doctor.md, doctor datastores section:

• the --repair flag description (it currently lists only namespace contamination cleanup and the format marker restore)
• exit codes
• text and JSON output examples for the new check, the repair preview and the executed repair

Update after the recovery PR for swamp-club#3955 merges.