when we do uv run nox -s build on a fresh machine, we first build as part of the uv run call (which installs the current project), and cibuildwheel (which gets called in the nox session) re-uses the binaries built in that initial step

Do we need to build in the uv run call? Doing the build only within cibuildwheel seems like a more obvious behavior to me, but I could be convinced otherwise. IIRC this at one point worked by just failing the build if cibuildwheel found existing binaries, perhaps we should go back to that approach?

Do we need to build in the uv run call?

Let me see if I can figure out how to avoid it. uv run installs the project by default, and I didn't see a handy --no-install-project flag for it the way there is with uv sync, but there's almost certainly a way to do this.

IIRC this at one point worked by just failing the build if cibuildwheel found existing binaries, perhaps we should go back to that approach?

That means nox -s build will fail on pretty much any developer machine, which seems undesirable. But maybe I can make that check only happen in the CI? Or, another option is the same genre would be to delete any build libraries first.

Okay, I think this is much better now. I have avoided building Core as part of uv run, and I have added a cibuildwheel check that will fail if the libraries have been built (but only on a CI runner). I also removed the part that added the MACOS_DEPLOYMENT_TARGET to every build session, since we probably don't care if we're building dev wheels, and it's one less thing to keep up to date.