refactor(scene): rename model_solid to solid_model

Match the `*_model` naming of the sibling render models (wire_model,
hatch_model, mesh_model, image_model). Also realign the related Scene
API for consistency: model_solids → solid_models, add_model_solid →
add_solid_model, register_model_solid → register_solid_model.

No behavior change; build and tests are green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/model/mod.rs

@@ -3,5 +3,5 @@ pub mod hatch_model;
 pub mod hatch_patterns;
 pub mod image_model;
 pub mod mesh_model;
-pub mod model_solid;
+pub mod solid_model;
 pub mod object;