feat(primitives_gallery): demo de gl_texture_load + asset PNG

Añade demos_gl_texture.cpp (carga assets/sample.png con la nueva
funcion gl_texture_load_cpp_gfx, muestra w/h/channels y sliders de
tint RGB + zoom UV via ImGui::ImageWithBg) y un PNG damero 256x256
generado para la demo.

Tambien registra apps/primitives_gallery como subdirectorio en
cpp/CMakeLists.txt (la app vive como WIP en master; aqui solo se
añade el hook de build).

Para integrarse, demos.h, main.cpp y CMakeLists.txt de la gallery
deben anadir respectivamente la declaracion gallery::demo_gl_texture(),
la entrada {"gl_texture", "gl_texture_load", "Gfx", &gallery::demo_gl_texture}
en k_demos[], y demos_gl_texture.cpp + cpp/functions/gfx/gl_texture_load.cpp +
cpp/vendor/stb/stb_image_impl.cpp + include de cpp/vendor/stb a sus sources.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/CMakeLists.txt

@@ -115,6 +115,11 @@ if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/apps/shaders_lab/CMakeLists.txt)
     add_subdirectory(apps/shaders_lab)
 endif()
 
+# --- Primitives Gallery ---
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/apps/primitives_gallery/CMakeLists.txt)
+    add_subdirectory(apps/primitives_gallery)
+endif()
+
 # --- Registry Dashboard (lives in projects/fn_monitoring/apps/) ---
 set(_DASH_DIR ${CMAKE_SOURCE_DIR}/../projects/fn_monitoring/apps/registry_dashboard)
 if(EXISTS ${_DASH_DIR}/CMakeLists.txt)