feat(primitives_gallery): demo de mesh_viewer (cubo procedural + .obj loader)

Genera cubo procedural in-line (mesh_obj_parse de string), permite
cargar .obj desde un text input absoluto. Botones: Reload cube,
Wireframe toggle, Load .obj. Status line con tris count y
instrucciones (drag to orbit, wheel to zoom).

issue 0029

cpp/apps/primitives_gallery/demos.h

@@ -34,6 +34,7 @@ void demo_candlestick();
 void demo_gauge();
 void demo_heatmap();
 void demo_table_view();
+void demo_mesh_viewer();      // issue 0029
 
 // --- Gfx ---
 void demo_shader_canvas();