feat(pdftract-4my): implement pdfium-render path behind full-render feature
Implements Phase 5.2.2: pdfium-render rendering path gated behind the
full-render Cargo feature, providing accurate rendering for complex PDFs
with overlapping images, image masks, soft masks, blend modes, and other
geometry the direct-compositing path cannot handle.
Changes:
- Add pdfium-render dependency gated under full-render feature
- Implement pdfium_path.rs module with thread-local PDFium instance
- Add render_page_via_pdfium() function for high-fidelity page rendering
- Add has_full_render() runtime detection helper
- Add ExtractionOptions.full_render field for runtime selection
- Re-export has_full_render from pdftract-core lib
Acceptance Criteria:
- ✅ cargo build --features ocr,serve,full-render produces binary
- ✅ cargo build --features ocr,serve does NOT pull in pdfium
- ✅ Runtime fallback: full_render=true without feature -> direct compositing
- ⚠️ Soft-mask fixtures: no fixtures added (testing infrastructure)
- ⚠️ Binary size CI gate: no CI infrastructure (infra task)
Refs:
- Plan section: Phase 5.2 full-render feature (line 1854)
- Bead: pdftract-4my
367a0f129e