Add structures and functions to record inline images (BI/ID/EI sequences)
as ImageXObject entries in a page's image list. This enables Phase 4.4
figure detection to correctly classify blocks containing only images.
Changes:
- Add InlineImageHeader struct for inline image metadata
- Add ImageBytesRef enum for image byte references
- Add ImageXObject struct unifying XObject and inline images
- Add collect_image_xobjects() to collect all images with bboxes
- Add parse_inline_image() to parse BI/ID/EI sequences
- Add compute_unit_square_bbox() for bbox computation from CTM
- Add comprehensive unit tests for all acceptance criteria
Acceptance criteria:
- Inline image with no CTM: bbox == [0,0,1,1] ✅
- Inline image with CTM 100 0 0 50 200 300: bbox == [200,300,300,350] ✅
- Page with 3 images: page_image_list has 3 entries with correct bboxes ✅
- Image mask: recorded with is_mask flag ✅
- Rotation normalization: handled via CTM ✅
Closes: pdftract-axcri
