feat(pdftract-byq): implement direct image compositing path (Phase 5.2.1)
Implements the default-feature image rendering path for scanned PDFs:
- Walk content stream operators and collect image XObjects with CTMs
- Decode image XObjects (JPEG, RGB, grayscale, CMYK) via Phase 1.5
- Composite images onto canvas using CTM-based pixel placement
- Support page rotation (0, 90, 180, 270 degrees)
- Handle Y-flip CTMs (common in PDFs)
- Emit IMG_SOFTMASK_UNSUPPORTED diagnostic for soft-masked images
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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