fix(release): refresh the newly published package index

[stefan-jansen]

Summary

• force uv to refresh the just-published ml4t-backtest index entry during post-release installation
• prevent a restored pre-publication cache from turning PyPI propagation into a false verification failure
• retain the immutable candidate, digest checks, clean installation, and documentation example gates

Evidence

• release 0.1.6 attempt 1 published matching artifacts, then failed because uv could not see the version while PyPI JSON already reported its digests
• rerunning only the failed verification job succeeded after propagation
• a local install reproduced the stale-cache failure before the refresh option and succeeded with it
• uv run pytest tests/contracts/test_release_workflow.py -q --no-cov: 8 passed
• focused Ruff and pre-commit checks passed

Closes #90

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