feat: integrate fork features with comprehensive Direct API methods and tests

## New Direct API Methods (Python 3.8 compatible)
- `split_pdf()` - Split PDFs into multiple documents by page ranges
- `duplicate_pdf_pages()` - Duplicate specific pages within a PDF
- `delete_pdf_pages()` - Remove specific pages from a PDF
- `add_page()` - Insert blank pages into PDFs
- `set_page_label()` - Apply custom labels to page ranges

## Comprehensive Integration Test Suite
- Added complete integration tests for all existing methods
- Added comprehensive tests for all new methods
- Tests cover both bytes return and file output scenarios
- Proper error handling and edge case testing
- Python 3.8+ compatible type hints throughout

## Quality Assurance
- All methods maintain Python 3.8+ compatibility
- Full type checking with mypy
- Comprehensive linting with ruff
- Detailed docstrings with examples
- Consistent error handling patterns

This integration successfully adopts the excellent fork functionality
while maintaining our quality standards and compatibility requirements.

Author: jdrhyne