plot_model_cartoon: kind="kde" with NDT-boundary-corrected kernels

Deliberately excluded from #1124: a Gaussian KDE on the per-side |rt| support smears predictive mass into rt < ndt — where the model assigns exactly zero — on the very plot whose point is displaying the NDT/boundary geometry. Doing it right needs boundary-corrected KDE (e.g. reflection at a per-draw ndt), which is new statistical machinery rather than a port of #1123's `kind="kde"`. Until then the cartoon intentionally has no `kind` parameter, so adding one later is backward compatible.

Start by comparing the plot_model_cartoon work described here with issues #1124 and #1123, especially the existing kind="kde" behavior. Investigate how per-draw ndt and per-side |rt| support are represented. Done means adding boundary-corrected KDE support without smearing mass below ndt, while preserving the cartoon's current behavior when no kind is supplied.

Written by the indexing model from the issue text.