This implements proper GIL release around all blocking extraction calls
so Python threads can run concurrently during PDF processing.
Changes:
- extract_py: Wrap extract_pdf call with py.allow_threads
- extract_stream: Release GIL during sleep between recv attempts
- Added Python multi-threading test to verify parallelism
- Added rlib to crate-type for unit test support
Acceptance criteria:
- PASS: GIL is released during extraction via py.allow_threads
- PASS: Multi-threading test added to Python test suite
- PASS: Code compiles and formatting verified
Closes: pdftract-1tswa
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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