fix: improve PDF layout generation for Direct track

Key fixes:
- Skip large vector_graphics charts (>50% page coverage) that cover text
- Fix font fallback to use NotoSansSC for CJK support instead of Helvetica
- Improve translated table rendering with dynamic font sizing
- Add merged cell (row_span/col_span) support for reflow tables
- Skip text elements inside table bboxes to avoid duplication

Archive openspec proposal: fix-pdf-table-rendering

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Co-Authored-By: Claude <noreply@anthropic.com>

openspec/changes/archive/2025-12-03-fix-pdf-table-rendering/proposal.md

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+# Change: Fix PDF Table Rendering Issues
+
+## Why
+OCR track PDF exports have significant table rendering problems:
+1. **Reflow PDF** (both translated and untranslated): Tables are misaligned due to missing row_span/col_span support
+2. **Translated Layout PDF**: Table borders disappear and text overlaps because it doesn't use the accurate `cell_boxes` positioning
+
+## What Changes
+- **Translated Layout PDF**: Adopt layered rendering approach (borders + text separately) using `cell_boxes` from metadata
+- **Reflow PDF Tables**: Fix cell extraction and add basic merged cell support
+- Ensure embedded images in tables are rendered correctly in all PDF formats
+
+## Impact
+- Affected specs: result-export
+- Affected code:
+  - `backend/app/services/pdf_generator_service.py`
+    - `_draw_translated_table()` - needs complete rewrite
+    - `_create_reflow_table()` - needs merged cell support