release.py now tags each release vX.Y.Z from the package version. The
old CalVer date tags stay readable as history. get_last_tag() prefers
the newest SemVer tag and falls back to the legacy CalVer tags for the
first SemVer release.
__release_rev_count__ records the commit count of the release-bump
commit. Immutable Nix builds carry no git history, so they derive the
"+N commits since release" display from this number and the flake's
revCount.
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