fix(picklescan): preserve POSIX ctime checks

[mldangelo-oai]

Summary

• keep Windows descriptor/path stat reconciliation unchanged
• preserve POSIX st_ctime in cross-view source identity checks
• add a regression test for same-size, same-mtime inode-reuse detection

Why

The Windows portability fix intentionally omits st_ctime because Windows reports it differently for descriptor and path stats. Applying that omission on POSIX weakens the source-read TOCTOU guard: a replacement that reuses the same inode and preserves size/mtime can only be distinguished by st_ctime.

Validation

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTHONPATH=/Users/mdangelo/.codex/worktrees/c184/modelaudit/packages/modelaudit-picklescan/src PROMPTFOO_DISABLE_TELEMETRY=1 /Users/mdangelo/code/modelaudit/.venv/bin/python -m pytest packages/modelaudit-picklescan/tests/test_call_graph_safe_spec_resolution.py -q
• PYTHONPATH=/Users/mdangelo/.codex/worktrees/c184/modelaudit/packages/modelaudit-picklescan/src /Users/mdangelo/code/modelaudit/.venv/bin/ruff check packages/modelaudit-picklescan/src/modelaudit_picklescan/call_graph.py packages/modelaudit-picklescan/tests/test_call_graph_safe_spec_resolution.py
• PYTHONPATH=/Users/mdangelo/.codex/worktrees/c184/modelaudit/packages/modelaudit-picklescan/src /Users/mdangelo/code/modelaudit/.venv/bin/ruff format --check packages/modelaudit-picklescan/src/modelaudit_picklescan/call_graph.py packages/modelaudit-picklescan/tests/test_call_graph_safe_spec_resolution.py
• PYTHONPATH=/Users/mdangelo/.codex/worktrees/c184/modelaudit/packages/modelaudit-picklescan/src /Users/mdangelo/code/modelaudit/.venv/bin/mypy packages/modelaudit-picklescan/src/modelaudit_picklescan/call_graph.py packages/modelaudit-picklescan/tests/test_call_graph_safe_spec_resolution.py

[github-actions]

Workflow run and artifacts

Performance Benchmarks

Compared 13 shared benchmarks with a regression threshold of 15%.
Status: 0 regressions, 0 improved, 13 stable, 0 new, 0 missing.
Aggregate shared-benchmark median: 4.160s -> 4.276s (+2.8%).

Workload	Benchmark	Target	Size	Files	Baseline	Current	Change	Status
direct-malicious-upload	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_direct_malicious_upload	malicious_reduce	52 B	1	188.4us	202.8us	+7.7%	stable
nested-payload-review	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_nested_payload_review[nested_raw]	nested_raw	78 B	1	240.8us	252.5us	+4.9%	stable
rejected-basic-auth-candidates	tests/benchmarks/test_scan_benchmarks.py::test_rejected_basic_auth_candidates_scan_linearly	-	371.1 KiB	1	2.329s	2.441s	+4.8%	stable
padded-multi-stream-upload	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_padded_multi_stream_upload	multi_stream_padded	4.1 KiB	1	299.4us	310.7us	+3.8%	stable
nested-payload-review	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_nested_payload_review[nested_base64]	nested_base64	98 B	1	259.9us	266.0us	+2.3%	stable
suspicious-pickle-intake	tests/benchmarks/test_scan_benchmarks.py::test_scan_suspicious_pickle_intake	suspicious-intake	183.8 KiB	4	149.16ms	148.01ms	-0.8%	stable
duplicate-heavy-registry	tests/benchmarks/test_scan_benchmarks.py::test_scan_duplicate_registry_snapshot	registry-snapshot	915.2 KiB	13	578.95ms	582.86ms	+0.7%	stable
single-checkpoint-preflight	tests/benchmarks/test_scan_benchmarks.py::test_scan_single_checkpoint_before_load	single_checkpoint.pkl	183.0 KiB	1	105.50ms	106.13ms	+0.6%	stable
chunked-upload-stream	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_chunked_upload_stream	chunked_stream	278.2 KiB	1	116.94ms	116.42ms	-0.4%	stable
nested-payload-review	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_nested_payload_review[nested_hex]	nested_hex	130 B	1	270.7us	271.7us	+0.4%	stable
mixed-model-repository	tests/benchmarks/test_scan_benchmarks.py::test_scan_release_candidate_repository	release-candidate	547.3 KiB	32	624.85ms	625.91ms	+0.2%	stable
warm-cache-rescan	tests/benchmarks/test_scan_benchmarks.py::test_scan_warm_cached_repository_rescan	release-candidate	547.3 KiB	32	141.11ms	141.31ms	+0.1%	stable
clean-training-checkpoint	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_clean_training_checkpoint	safe_large	278.2 KiB	1	113.61ms	113.60ms	-0.0%	stable

[ianw-oai]

Small, focused POSIX ctime fix; the Windows-specific behavior is preserved.