David debugged: Investigated prefetch_state 'failed' contradicting six successful prefetches

David noticed Kerra's lecture-notes pipeline was reporting prefetch_state='failed' even though prefetch_metrics showed attempted=6, completed=6, failed=0. When the agent reported this as a prefetch success, David didn't accept it:

i dont believe you. go and see what the primary files are that got prefetched and see if they were actually downloaded to s3

Further investigation revealed the field had been overwritten by the downstream lecture-generation failure reason — meaning prefetch_state was being reused as a general error sink rather than a prefetch-only status. David's insistence on empirical verification forced the agent to dig past the misleading surface metric and find the actual field semantics bug.