Add Marker parser adapter (LLM-enhanced batch processing)

[kiyeonjeon21]

What

Marker supports batch PDF processing with optional LLM enhancement. Adapter exists at src/parsemux/parsers/marker.py but needs real-world testing.

How

1. pip install parsemux[marker]
2. Test with --use-llm flag
3. Verify BYOK key flow works end-to-end
4. Test batch processing performance

Note

Marker has cc-by-nc-sa license (free for < $5M revenue). Document this clearly.