Add callback-based streaming API to pdftract-core and PyO3 bindings that
return a Python iterator yielding page dicts incrementally. This provides
memory-efficient extraction for large PDFs via the iterator protocol.
Core changes:
- Add extract_pdf_streaming() callback-based function to pdftract-core
- Export extract_pdf_streaming in lib.rs
PyO3 bindings:
- Add StreamIterator PyClass with __iter__/__next__ methods
- Add extract_stream_fn() spawning background thread with mpsc channel
- Add *Frame types for efficient Python dict serialization
- Integrate into pdftract Python module
Closes: pdftract-bnba5
