feat(shaders_lab): add gl_loader + Windows cross-compile

- cpp/functions/gfx/gl_loader.{h,cpp,md}: mini loader para OpenGL 2.0+
  (Linux no-op via GL_GLEXT_PROTOTYPES, Windows wglGetProcAddress)
- Portar gl_shader/gl_framebuffer/fullscreen_quad/shader_canvas al loader
- CMakeLists: WIN32_EXECUTABLE para lanzar sin consola en Windows
- apps/shaders_lab/shaders_lab.exe: binario PE32+ precompilado

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

cpp/apps/shaders_lab/CMakeLists.txt

@@ -1,5 +1,6 @@
 add_imgui_app(shaders_lab
     main.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/gl_loader.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/gl_shader.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/gl_framebuffer.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/fullscreen_quad.cpp
@@ -9,3 +10,8 @@ add_imgui_app(shaders_lab
 target_include_directories(shaders_lab PRIVATE
     ${CMAKE_CURRENT_SOURCE_DIR}
 )
+
+if(WIN32)
+    # GUI app: sin consola al lanzar (subsystem:windows / -mwindows)
+    set_target_properties(shaders_lab PROPERTIES WIN32_EXECUTABLE TRUE)
+endif()