Thank you all for your answers. I ended up implementing webpack. The docs on running w/o a build system are not yet super clear and setting up webpack was half an hour of work.
Thank you @tleish – this approach worked for me when trying to use both Stimulus and Turbo via Skypack. Although oddly (?) given @kfk’s experience, it doesn’t seem to be necessary to do the same for Turbo at this point.
<script type="module">
import Turbo from 'https://cdn.skypack.dev/@hotwired/turbo';
import * as Stimulus from 'https://cdn.skypack.dev/stimulus';
window.Stimulus = Stimulus;
</script>
@leoc - you are correct, starting with v7.0.0-beta.6 release it now sets window.Turbo “automatically on import to accommodate the needs of the native mobile adapters and ease of use”