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/CMakeLists.txt

@@ -6,6 +6,7 @@ add_imgui_app(primitives_gallery
     demos_graph.cpp
     demos_gfx.cpp
     demos_text_editor.cpp
+    demos_gl_texture.cpp
     # text_editor + file_watcher (issue 0025)
     ${CMAKE_SOURCE_DIR}/functions/core/text_editor.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/file_watcher.cpp
@@ -46,9 +47,13 @@ add_imgui_app(primitives_gallery
     ${CMAKE_SOURCE_DIR}/functions/gfx/gl_framebuffer.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/fullscreen_quad.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/shader_canvas.cpp
+    # 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
 )
 target_include_directories(primitives_gallery PRIVATE
     ${CMAKE_SOURCE_DIR}/vendor/imgui_text_edit
+    ${CMAKE_SOURCE_DIR}/vendor/stb
 )
 
 if(WIN32)