Debug draw height

Add a debug view to draw model height (can be just a ribbon from base upwards). Perhaps this can be added to `/debugcolvol` to avoid adding more modes (in particular Upspring shows height and colvol together).

Start by locating the /debugcolvol debug view and reading how it draws collision volumes. Add a model-height visualization there, such as a ribbon from the base upward, and verify that height and collision volume can be viewed together as described for Upspring.

Written by the indexing model from the issue text.