Recognise PDFs in clean.Kind (declared types and %PDF- sniffing on
octet-stream/bodyless bodies), give PDF responses the 100 MB body cap and
120 s deadline while web pages keep 5 MB / 20 s, and add FetchIf, which
sends If-None-Match / If-Modified-Since and maps a 304 to a Resource with
Status 304 and an empty Body. Fetch and FetchIf share one doFetch path for
guard, client, limits and error mapping.
Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>
