build: replace printpdf 0.7 with own PDF writer on lopdf 0.46

printpdf 0.7 pinned lopdf 0.31; newer printpdf versions are a complete
rewrite. The export now uses src/pdf_writer.rs, a small writer on top of
lopdf 0.46 with the subset of the printpdf API the export used: pages with
layers (optional content groups), paths, embedded TrueType fonts (Type0,
Identity-H, ToUnicode), images and bookmarks. Bookmark titles are written
as UTF-16BE directly, so the post-processing step for them is gone.
pdf_merge uses lopdf 0.46 as well.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib.rs

@@ -21,6 +21,7 @@ pub mod page_export;
 pub mod paths;
 pub mod pdf_doc;
 pub mod pdf_merge;
+pub mod pdf_writer;
 pub mod platform;
 pub mod rnote;
 pub mod ruler;