Fix flaky test tests/test_defect_attribution.py::test_defect_attribution_same_files_only_neither_moves_nor_blocks

The merge lane saw `tests/test_defect_attribution.py::test_defect_attribution_same_files_only_neither_moves_nor_blocks` fail and then pass on a rerun of the same tree. - Test: `tests/test_defect_attribution.py::test_defect_attribution_same_files_only_neither_moves_nor_blocks` - Tree: `27a2b09202731bb41ee64b1541d156a75ff52c9d` - Lane task: 13975 - Flake record: 01a1a783-79f6-7a47-9d7a-1b6b6bab8afb Make it deterministic; do not delete it unless it tests nothing.

Fixed flake: origin fixture's PR number was hash-random (100–1099) and could equal evidence PR 901 (skipped as origin's own). Pinned to 41; collision reproduced, 83 tests + quick gate green. PR #7982 queued. Memory: none applied.