feat(shaders_lab): scaffold C++ app with GLSL live-reload canvas

- cpp/functions/gfx: gl_shader, gl_framebuffer, fullscreen_quad, shader_canvas
- cpp/apps/shaders_lab: main + 3 seed shaders (plasma, circle, checker)
- ImGui docking layout: Code | Canvas | Controls

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cpp/functions/gfx/gl_framebuffer.h

@@ -0,0 +1,16 @@
+#pragma once
+
+namespace fn::gfx {
+
+struct Framebuffer {
+    unsigned int fbo = 0;
+    unsigned int tex = 0;  // GL_RGBA8, clamp, linear
+    int width = 0;
+    int height = 0;
+};
+
+void fb_init(Framebuffer& f);                 // crea fbo+tex 1x1 iniciales
+void fb_resize(Framebuffer& f, int w, int h); // no-op si w,h iguales
+void fb_destroy(Framebuffer& f);
+
+} // namespace fn::gfx