Canonical previews for the kit's own components.

These live in the kit, not in each host, because /design promises to be identical across every JobyKit consumer. Previously the previews for kit components were generated into <App>Web.DesignPreviews and owned by the host, so they diverged the moment anyone edited them — and they never picked up components added in a later kit release. Two apps on the same kit version showed different buttons for the same <.button>, with different prose beside them.

Hosts write previews for their components. The kit writes previews for the kit's.