Join multiple streams in a page's /Contents array with a space (b' ') rather than a newline (b'\n') in autotag::content_bytes, alt::page_content, and content::page_content so a % comment at the end of one stream continues through the start of the next stream per ISO 32000-2 section 7.8.2.
Update alt::artifact_marked_content to join page content streams with pdfcraft_content::Pieces::join, parse operators across the joined stream, and rewrite only matching BDC spans via Pieces::splice so operators split across stream boundaries and cross-stream comments are preserved.
Fixes#3217
jonathanwxh-cell