Short commit SHA, <sha>-dirty, or 'unknown'.
ISO-8601 UTC build instant, or 'unknown'.
False when no stamp was injected — a dev/test/embedder context rather than
a build product. Callers reporting a version to a user should say so
instead of presenting 'unknown' as if the build were broken.
Semver-normalized CalVer, e.g.
2026.6.5, or'unknown'.