ci: reduce GitHub Actions runner usage

[yu-iskw]

Summary

Keep the complete supported Python range as pre-merge coverage while removing compatibility orchestration from GitHub Actions.

• noxfile.py owns supported versions and uv-backed isolated environments while preserving the project's Flit development-install semantics
• dev/test_all.sh is a tiny tagged-Nox wrapper; make test-all is the local entrypoint
• GitHub Actions has no Python-version matrix or shell session-name construction
• installation/import validation runs once on the primary Python environment
• cancellation remains; triggered validation is not suppressed for draft PRs

Future supported-version changes are localized to Nox.

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