- Add IMarkdownConverter interface with heuristic-based conversion
- Implement MarkdownConverter with optional LLM enhancement via DI
- Support heading detection (Markdown, ALL CAPS, numbered sections)
- Support list conversion (bullets, unicode bullets, numbered)
- Support table detection and formatting
- Support code block preservation with language hints
- Support image placeholder conversion
- Add whitespace normalization with Windows/Unix line ending support
- Add 37 unit tests for comprehensive coverage
- Clean up debug Console.WriteLine in MultiModal readers