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/CMakeLists.txt

@@ -8,6 +8,7 @@ add_imgui_app(primitives_gallery
     demos_text_editor.cpp
     demos_gl_texture.cpp
     demos_extras.cpp
+    demos_mesh.cpp
     # text_editor + file_watcher (issue 0025)
     ${CMAKE_SOURCE_DIR}/functions/core/text_editor.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/file_watcher.cpp
@@ -55,6 +56,11 @@ add_imgui_app(primitives_gallery
     # gl_texture_load (issue 0026) + stb_image
     ${CMAKE_SOURCE_DIR}/functions/gfx/gl_texture_load.cpp
     ${CMAKE_SOURCE_DIR}/vendor/stb/stb_image_impl.cpp
+    # mesh_viewer stack (issue 0029)
+    ${CMAKE_SOURCE_DIR}/functions/gfx/mesh_obj_load.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/mesh_gpu.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/orbit_camera.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/mesh_viewer.cpp
 )
 target_include_directories(primitives_gallery PRIVATE
     ${CMAKE_SOURCE_DIR}/vendor/imgui_text_edit