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/main.cpp

@@ -57,6 +57,7 @@ static const DemoEntry k_demos[] = {
     {"graph_viewport",  "graph_viewport",  "Viz",  &gallery::demo_graph},
     // Gfx (shaders_lab core)
     {"shader_canvas",   "shader_canvas",   "Gfx",  &gallery::demo_shader_canvas},
+    {"gl_texture",      "gl_texture_load", "Gfx",  &gallery::demo_gl_texture},
 };
 static constexpr int k_demo_count = sizeof(k_demos) / sizeof(k_demos[0]);