feat(imgui): embeddable plot widget and demo

examples/CMakeLists.txt

@@ -8,3 +8,16 @@ if(TARGET plot::glfw)
   target_link_libraries(plot_glfw_demo PRIVATE plot::glfw Threads::Threads)
   plot_apply_warnings(plot_glfw_demo)
 endif()
+
+if(TARGET plot::imgui)
+  # The demo owns the window, the ImGui context and the frame loop itself, so it
+  # links the toolkit and its backends directly; the adapter is only the widget.
+  plot_find_glfw(plot_imgui_demo_glfw_dep)
+  add_executable(plot_imgui_demo imgui_demo.cpp)
+  target_link_libraries(plot_imgui_demo PRIVATE
+    plot::imgui
+    imgui
+    ${plot_imgui_demo_glfw_dep}
+    Threads::Threads)
+  plot_apply_warnings(plot_imgui_demo)
+endif()