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/CMakeLists.txt

@@ -99,6 +99,11 @@ if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/apps/chart_demo/CMakeLists.txt)
     add_subdirectory(apps/chart_demo)
 endif()
 
+# --- Shaders Lab ---
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/apps/shaders_lab/CMakeLists.txt)
+    add_subdirectory(apps/shaders_lab)
+endif()
+
 # --- Registry Dashboard (lives in projects/fn_monitoring/apps/) ---
 set(_DASH_DIR ${CMAKE_SOURCE_DIR}/../projects/fn_monitoring/apps/registry_dashboard)
 if(EXISTS ${_DASH_DIR}/CMakeLists.txt)