refactor(render): unify scene rendering paths

Route model, paper, viewport and nested block content through one traversal while preserving leaf renderers. Carry decoded material state into mesh shading and harden solid geometry conversion.

src/scene/model/mesh_model.rs

@@ -84,8 +84,8 @@ pub enum CurvedGen {
         phi_min: f32,
         phi_max: f32,
     },
-    /// Torus: the two extreme circles of the tube (outer/inner), clipped to the
-    /// revolution window the face covers.
+    /// Torus: view-dependent tube silhouette, clipped to both parametric
+    /// windows the face covers.
     Torus {
         center: [f32; 3],
         center_low: [f32; 3],
@@ -97,6 +97,9 @@ pub enum CurvedGen {
         phi_min: f32,
         phi_span: f32,
         full: bool,
+        theta_min: f32,
+        theta_span: f32,
+        theta_full: bool,
     },
 }