The pdf module is the whole of M1 except the debug overlay, which needs
detection results to draw.
Source type is detected per PDF by looking for a page-covering image,
and is always reported for confirmation rather than applied silently
(ADR 0004). Rasters are extracted via Pixmap(doc, xref) rather than by
decoding extract_image() bytes, because MuPDF handles JBIG2 and CCITT
scans that no image library will.
Scanned pages are extracted at the embedded image's native resolution;
only vector pages are rendered, at 600 DPI. Verified against the corpus:
Elaman nalka (vector) renders 4959x7017, Ketun joululaulu (scan) loads
1653x2332, Engel (scan) 2552x3504 — and Engel's page 2 is 2480 wide
where page 1 is 2552, so scan width varies within one PDF.
tests/test_pdf.py builds its own PDFs so the check runs without corpus
files, which are copyrighted and gitignored.
Closes#1, #2, #3
