v6.0.4

Hotfix — Re-cut to escape v6.0.3 duplicate-run race

v6.0.3 was tagged once but GitHub Actions kicked off two concurrent pipeline runs against the same tag. The first run made it through verify-testpypi-install cleanly (the smoke-import fix from v6.0.3 worked), but the duplicate run failed at Publish to TestPyPI with 400 File already exists, and the first run's Publish to PyPI deployment was rejected during the manual-approval step (operator saw the chaos and stopped it). v6.0.4 carries no code or workflow changes vs v6.0.3 — it's a clean re-cut so the pipeline runs once.

Operator impact: zero behavior change. The wheel is identical to v6.0.3 (which is identical to v6.0.0–v6.0.2). v6.0.3 sits on TestPyPI as a build artifact but never promoted.