Add PNG raster fallback for SVG receipts when font outlines are
unavailable (OCR-sourced glyphs or Type 3 fonts).
- New ocr_fallback.rs module with 150 DPI rendering
- Integrate with SVG generator via GlyphSource enum
- Add data-source="ocr" attribute to OCR-generated SVGs
- Graceful degradation without full-render feature
Closes: pdftract-4yspv
