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.cpp

@@ -0,0 +1,30 @@
+#define GL_GLEXT_PROTOTYPES
+#include <GL/gl.h>
+#include <GL/glext.h>
+
+#include "gfx/fullscreen_quad.h"
+
+namespace fn::gfx {
+
+void quad_init(Quad& q) {
+    glGenVertexArrays(1, &q.vao);
+    glGenBuffers(1, &q.vbo);
+    // Vertex shader generates positions from gl_VertexID — VBO stays empty.
+    glBindVertexArray(q.vao);
+    glBindBuffer(GL_ARRAY_BUFFER, q.vbo);
+    glBindBuffer(GL_ARRAY_BUFFER, 0);
+    glBindVertexArray(0);
+}
+
+void quad_draw(const Quad& q) {
+    glBindVertexArray(q.vao);
+    glDrawArrays(GL_TRIANGLES, 0, 6);
+    glBindVertexArray(0);
+}
+
+void quad_destroy(Quad& q) {
+    if (q.vao) { glDeleteVertexArrays(1, &q.vao); q.vao = 0; }
+    if (q.vbo) { glDeleteBuffers(1, &q.vbo); q.vbo = 0; }
+}
+
+} // namespace fn::gfx