scikit-build

Test a downstream project against scikit-build-core to check whether the current checkout regresses it versus a released baseline. Builds the project twice per mode — once against a released tag (baseline, via a git worktree) and once against the working tree — in both a normal wheel build and an editable install, then reports parity, wheel-content, and warning diffs. Use this whenever the user wants to test/build/try a downstream or real-world project against their changes, check that a fix or PR doesn't break downstream, run `nox -s downstream`, compare current-vs-released behavior, reproduce a downstream build issue, or validate against projects in docs/data/projects.toml — even if they only name a project (e.g. "test iminuit", "does gemmi still build").