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/shader_canvas.h

@@ -0,0 +1,28 @@
+#pragma once
+#include <string>
+#include "gfx/gl_framebuffer.h"
+#include "gfx/gl_shader.h"
+#include "gfx/fullscreen_quad.h"
+
+namespace fn::gfx {
+
+struct ShaderCanvas {
+    Framebuffer fb;
+    Quad quad;
+    unsigned int program = 0;
+    bool initialized = false;
+};
+
+// Inicializa recursos GL (idempotente).
+void canvas_init(ShaderCanvas& c);
+
+// Sustituye el programa activo (borra el anterior). Acepta program=0 para pantalla en negro.
+void canvas_set_program(ShaderCanvas& c, unsigned int program);
+
+// Renderiza el shader al FBO y dibuja la textura resultante como contenido del panel ImGui.
+// Llamar DENTRO de un ImGui::Begin/End. Ocupa GetContentRegionAvail().
+void canvas_render(ShaderCanvas& c, float time_seconds);
+
+void canvas_destroy(ShaderCanvas& c);
+
+} // namespace fn::gfx