Remove the mandatory `?v=` from the renderer fetch

Today, in preview mode the renderer is downloaded always, in local and remote scenes (even if you've already downloaded). Also, it's not caching the option with renderer-version, so this is not a graceful workaround. The only useful case is when the renderer endpoint it's a dynamic and no deterministic version.
I think that this aplies to `kernel` too.

No files, tests, or entry points are named. Trace the preview-mode renderer fetch and the corresponding kernel handling, then determine how renderer versions are cached. Done means the renderer is not forced to use ?v= while dynamic endpoints remain supported and versioned downloads still cache correctly.

Written by the indexing model from the issue text.