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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

openspec/changes/archive/2025-12-03-fix-pdf-table-rendering/specs/result-export/spec.md

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+## MODIFIED Requirements
+
+### Requirement: Translated Layout PDF Generation
+The system SHALL generate layout-preserving PDFs with translated content that maintain accurate table structure.
+
+#### Scenario: Table with accurate borders
+- **GIVEN** an OCR result with tables containing `cell_boxes` metadata
+- **WHEN** generating translated layout PDF
+- **THEN** table cell borders SHALL be drawn at positions matching `cell_boxes`
+- **AND** translated text SHALL be rendered within each cell's bounding box
+
+#### Scenario: Text overflow handling
+- **GIVEN** translated text longer than original text
+- **WHEN** text exceeds cell bounding box
+- **THEN** the system SHALL reduce font size (minimum 8pt) to fit content
+- **OR** truncate with ellipsis if minimum font size is insufficient
+
+#### Scenario: Embedded images in tables
+- **GIVEN** a table with `embedded_images` in metadata
+- **WHEN** generating translated layout PDF
+- **THEN** images SHALL be rendered at their original positions within the table
+
+### Requirement: Reflow PDF Table Rendering
+The system SHALL generate reflow PDFs with properly structured tables including merged cell support.
+
+#### Scenario: Basic table rendering
+- **GIVEN** an OCR result with table cells containing `row`, `col`, `content`
+- **WHEN** generating reflow PDF
+- **THEN** cells SHALL be grouped by row and column indices
+- **AND** table SHALL render with visible borders
+
+#### Scenario: Merged cells support
+- **GIVEN** table cells with `row_span` or `col_span` greater than 1
+- **WHEN** generating reflow PDF
+- **THEN** the system SHALL apply appropriate cell spanning
+- **AND** merged cells SHALL display content without duplication
+
+#### Scenario: Column width calculation
+- **GIVEN** a table with `cell_boxes` metadata
+- **WHEN** generating reflow PDF
+- **THEN** column widths SHOULD be proportional to original cell widths