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/main.cpp

@@ -61,6 +61,7 @@ static const DemoEntry k_demos[] = {
     {"gauge",           "gauge",           "Viz",  &gallery::demo_gauge},
     {"heatmap",         "heatmap",         "Viz",  &gallery::demo_heatmap},
     {"table_view",      "table_view",      "Viz",  &gallery::demo_table_view},
+    {"mesh_viewer",     "mesh_viewer",     "Viz",  &gallery::demo_mesh_viewer},
     // Gfx (shaders_lab core)
     {"shader_canvas",   "shader_canvas",   "Gfx",  &gallery::demo_shader_canvas},
     {"gl_texture",      "gl_texture_load", "Gfx",  &gallery::demo_gl_texture},      // wave 1