PyMuPDF: put a text stamp on rotated or cropped PDF pages without it coming out sideways/upside down

Find the spot in visual coordinates (render with page.get_pixmap() and search for white space), then:

• Simple page (rotation == 0, cropbox == mediabox, origin at 0,0): draw directly.
• Otherwise draw the stamp upright on a small temporary page and place it with show_pdf_page, mapping the visual rect through the derotation matrix and passing the page rotation:

def stamp_page(page, rect, draw):   # rect in visual (rotated) coordinates
    if page.rotation == 0 and page.rect.x0 == 0 and page.rect.y0 == 0 and page.cropbox == page.mediabox:
        page.wrap_contents()          # isolate from graphics state the drawing left open
        draw(page, rect)
    else:
        tmp = pymupdf.open(); tp = tmp.new_page(width=rect.width + 4, height=rect.height + 4)
        draw(tp, pymupdf.Rect(2, 2, 2 + rect.width, 2 + rect.height))
        page.wrap_contents()
        grown = pymupdf.Rect(rect.x0 - 2, rect.y0 - 2, rect.x1 + 2, rect.y1 + 2)
        page.show_pdf_page(grown * page.derotation_matrix, tmp, 0,
                           rotate=page.rotation, keep_proportion=False)

The first attempt used rotate=-page.rotation; that produced upside-down text on 90/270 pages. rotate=page.rotation is correct together with rect * page.derotation_matrix.

Before saving, call doc.subset_fonts() if you embedded a CJK TTF with insert_font; it shrank a ~10 MB font to a few KB.