Flag audit fixes: tri-state CLI bools, marginalia keep flags, app mode selector

- printer.py: only default-False bools render as CLI flags, so tri-state
  options like --ocr_inline_math accept explicit true/false
- MarginaliaProcessor respects keep_pageheader/pagefooter_in_output
- streamlit + extraction apps expose the balanced/fast mode selector
- Docstring/help fixes: stale detection-model references, keep_chars
  pdftext-only note, disable_multiprocessing scope, LLM redo_inline_math
  cross-references

Claude-Session: https://claude.ai/code/session_01SxGJdyccsAdKbjNAth35MA