Implement the foundation for the --highlight DIR feature that writes
annotated PDFs with /Highlight annotations for grep matches.
Changes:
- Create highlight.rs module with grouping, annotation dict creation
- Add /Highlight annotation with proper /QuadPoints (BL, BR, TR, TL per spec)
- Implement output filename collision handling with -1/-2 suffixes
- Make progress module conditional on grep feature to fix compilation
- Fix borrow issues in worker.rs
The write_single_highlighted_pdf() function currently does a simple
file copy as a placeholder. The full incremental update implementation
(xref parsing, object allocation, trailer update) is left for a follow-up
bead due to complexity.
Closes: pdftract-22q8e (partial - foundation only, full incremental update TODO)
