fix: keep PyMuPDF asset probe from hanging or dropping results

[suguanYang]

Summary

• Skip find_tables and drawing clustering on image/vector-dense pages so _probe_assets_worker stays under the 300s child timeout (production lipoprotein PDF page 2 has ~114k drawings).
• Flush the multiprocessing result queue in the child after put(), and retry once on exitcode=0 with an empty queue so a healthy probe is not reported as SUBPROCESS_CRASH.

Test plan

• uv run pytest tests/unit/test_probe_dense_visual_assets.py tests/unit/test_pymupdf_subprocess_queue.py tests/unit/test_probe_visible_text.py
• Local production path: lipoprotein PDF probe finished in 21.8s (previously timed out at 300s)
• Local production path: nucleic PDF probe succeeded 3/3
• Staging re-parse of job_6265564f67e8 and job_83eb3d464064 after merge

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