Add pdf_doc/pdf_page tables plus PDFEntry, PDFKey, PutPDF (one transaction,
old pages deleted first), GetPDF (page-ordered, updates last_hit) and TouchPDF
(304 revalidation moves only fetched_at). PDF documents count toward Size() and
are evicted whole by oldest use; PDFH/PDFM counters added. evict() now breaks
last_hit ties by rowid so same-second inserts evict in insertion order.
Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>
