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/app/cmd_result.rs

@@ -134,7 +134,7 @@ impl OpenCADStudio {
             CmdResult::CommitSolid { entity, solid } => {
                 let label = self.history_label_from_active_cmd(i, "SOLID");
                 self.push_undo_snapshot(i, label);
-                self.add_model_solid(entity, *solid);
+                self.add_solid_model(entity, *solid);
                 self.tabs[i].dirty = true;
                 self.tabs[i].scene.clear_preview_wire();
                 self.tabs[i].active_cmd = None;