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/apps/shaders_lab/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_imgui_app(shaders_lab
+    main.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/gl_shader.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/gl_framebuffer.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/fullscreen_quad.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/shader_canvas.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/fps_overlay.cpp
+)
+target_include_directories(shaders_lab PRIVATE
+    ${CMAKE_CURRENT_SOURCE_DIR}
+)