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>

README.md

@@ -181,6 +181,7 @@ src/
 ├── assets.rs       # Images and PDFs in the notebook folder
 ├── pdf_doc.rs      # Reading and rendering PDFs (hayro)
 ├── pdf_merge.rs    # Underlaying imported PDF pages on export
+├── pdf_writer.rs   # Small PDF writer for the export (lopdf)
 ├── import.rs       # Import as a new notebook (shared parts)
 ├── xopp.rs         # Import of Xournal++/Xournal notebooks
 ├── rnote.rs        # Import of Rnote notebooks
@@ -191,7 +192,7 @@ src/
 ├── disk_watch.rs   # Taking over changes from other devices (sync clients)
 ├── library.rs      # Notebook folders (library.json)
 ├── autosave.rs     # Saving in the background
-├── export.rs       # PDF export (printpdf)
+├── export.rs       # PDF export
 ├── page_export.rs  # Page as PNG/SVG, selection as image
 ├── file_picker.rs  # Built-in file picker
 ├── settings.rs     # Settings (~/.config/ink-notes/ui.json)