URNs provide an abstraction and a single place for e.g updating references to external resources when switching between a CDN and a local resource, or when a resource should be updated. Instead of adding e.g. a CDN url via tempate::head::add_script, one can add an URN and control its content from a single place.
template::head::includes -container .. -parts ...
if an included resource is encountered during rendering of the page, it is ignored. The compound resource management is aligned resource flushing (when a compound resource is flushed, it stop shadowing potentially other resources. An additional option could be to add an "-parts" flag to add_link/add_script.
added simple regression test for basic functionality