feat(primitives_gallery): wire gl_texture_load demo

Tras integrar 0025 (que trajo la app entera) y 0026 (que solo añadio
demos_gl_texture.cpp + assets/sample.png + cpp/functions/gfx/gl_texture_load.*),
falta wire-up de la entrada nueva en los archivos compartidos del gallery.

- demos.h: declarar gallery::demo_gl_texture()
- main.cpp: entrada en k_demos[] con id "gl_texture", category "Gfx"
- CMakeLists.txt: añadir demos_gl_texture.cpp, gl_texture_load.cpp,
  stb_image_impl.cpp, e include dir vendor/stb

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

cpp/apps/primitives_gallery/demos.h

@@ -30,6 +30,7 @@ void demo_graph();
 
 // --- Gfx ---
 void demo_shader_canvas();
+void demo_gl_texture();
 
 // --- Core (combined demo: text_editor + file_watcher) ---
 void demo_text_editor();