The recorded run's stored prompt now links in one transaction with the file's
link rows, so a file linked to another prompt is a conflict instead of a
second link. The scan's stores run through survive_library_switch. A single
file asks history once (a one-file _BatchHistory), the scan passes
in_output=False and reads the file size only for files with prompt
metadata. A conflict row reads "Kept: already linked to a different
prompt". TESTING.md records the rules.