feat(build): install/export rules, umbrella header, verified consumer

adapters/imgui/CMakeLists.txt

@@ -52,3 +52,12 @@ target_include_directories(plot_imgui PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/..")
 # consumer of plot::imgui does not inherit the toolkit's headers.
 target_link_libraries(plot_imgui PUBLIC plot::core PRIVATE imgui ${plot_imgui_glfw_dep})
 plot_apply_warnings(plot_imgui)
+
+# Not installed and not exported, and not because of a discovery fallback: ImGui
+# is *always* fetched here, so there is no ImGui package for an installed config
+# file to find. Exporting plot::imgui would name the fetched `imgui` target in its
+# link interface and leave every consumer of `find_package(plot)` unable to
+# configure -- so plot::imgui is a build-tree target only, and a consumer that
+# wants it adds the adapter's source or fetches the same pinned tag itself.
+plot_record_adapter_export(plot_imgui FALSE
+  "ImGui is supplied by FetchContent, so find_package(plot) cannot promise it to a consumer" "")