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/fullscreen_quad.h

@@ -0,0 +1,14 @@
+#pragma once
+
+namespace fn::gfx {
+
+struct Quad {
+    unsigned int vao = 0;
+    unsigned int vbo = 0;
+};
+
+void quad_init(Quad& q);
+void quad_draw(const Quad& q);
+void quad_destroy(Quad& q);
+
+} // namespace fn::gfx