selection outline renders as wireframe

This is most noticeable on polygons with alpha blending.
Example Polygons:
* `data/model/figure/bayonetta_d_F` TopN
- [x] Scaled model with stencil testing for outlines
- [ ] Fix visual artifacts on corners
- [ ] Fix alpha when taking screenshots
- [ ] Scale thickness relative to screen size

Reproduce the artifact using data/model/figure/bayonetta_d_F TopN, focusing on polygons with alpha blending and the existing scaled-model stencil outline path. Compare the rendering at corners and during screenshots, then verify the remaining checklist items: clean corners, correct screenshot alpha, and screen-relative thickness.

Written by the indexing model from the issue text.