Advance development to 0.8.0.dev0 - #57

Summary

• record the verified v0.7.1 release commit, workflows, public hashes, and successful OIDC publication
• advance package and documentation development state to 0.8.0.dev0
• add the v0.8.0 release-gate placeholder without changing any frozen result

Validation

• python scripts/release_state.py --check
• python scripts/build_pypi_readme.py --check
• python scripts/check_markdown_links.py
• focused release-state and packaging tests: 22 passed
• ruff check and format check
• frozen benchmark/evidence/preregistration diff against v0.7.1: clean

Published v0.7.1 evidence

• release run: https://github.com/DaoyuanLi2816/mini-verl/actions/runs/31566663507
• wheel SHA-256: 7d29669eaf53de0fd9b3056f3d00ad1d61c990fbe4f6cc97ccaf8e628c60e785
• sdist SHA-256: 8131faee22e8b668bb0ff010f92ffafd9224aa046a017ad6f85fe7774301dea1