Route "pdf" through clean.Kind in doFetch/cleanFetched: extract under the pool
(keyed by caller class + document hash, one per caller), map the five pdftext
errors plus a pool panic to 422, ErrBusy to 503+Retry-After 10, ErrCallerBusy to
429, and answer with the whole text or the map. Adds RetryAfter to fetch.Error
and the matching header. No cache, pages or grep — those are task 07.
Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>
