feat(pdf): add font-based heading detection and refactor PDF/Markdown parsing

[qin-ctx]

Description

Add font-based heading detection for PDFs, refactor PDF bookmark extraction, and clean up Markdown parser. Remove auto-group directory logic. Translate Chinese comments to English.

Related Issue

fixed #393

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• PDFParser: Add _detect_headings_by_font() for font-size-based heading detection with configurable thresholds
• PDFParser: Add heading detection strategy (heading_detection: bookmarks | font | auto | none) with bookmarks-to-font fallback
• PDFParser: Refactor _extract_bookmarks() for clarity, translate all Chinese comments to English
• PDFConfig: Add heading_detection, font_heading_min_delta, max_heading_levels config options
• MarkdownParser: Remove auto-group directory logic (max_children_per_dir, _auto_group_sections())
• ParserConfig: Remove max_children_per_dir field

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

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