fix(batch): drain finalize after fail-fast; add tests

[AEliu]

Summary

• fix(batch): allow queued finalize to drain after fail-fast on finalize in pipeline mode
• test(batch): add coverage for fail-fast on finalize verifying no new downloads launch while queued finalize completes, and in-flight download's prepared finalize still succeeds

Details

• manager.py: break only when stop_batch is set AND both slots are idle AND staged_queue is empty, so finalize draining can complete
• tests: deterministic ordering via Events to avoid flakes; no network/filesystem

Test plan

• uv run ruff check .
• uv run mypy --hide-error-context --pretty --strict src/artx
• uv run pytest -q

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