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/functions/gfx/shader_canvas.cpp

@@ -1,7 +1,4 @@
-#define GL_GLEXT_PROTOTYPES
-#include <GL/gl.h>
-#include <GL/glext.h>
-
+#include "gfx/gl_loader.h"
 #include "gfx/shader_canvas.h"
 #include "imgui.h"
 
@@ -9,6 +6,7 @@ namespace fn::gfx {
 
 void canvas_init(ShaderCanvas& c) {
     if (c.initialized) return;
+    gl_loader_init();
     fb_init(c.fb);
     quad_init(c.quad);
     c.initialized = true;