A lightweight registry that maps canonical atlas ids and aliases to their loader functions. The registry makes it possible to:
* enumerate available atlases ([list_atlases()]), * look up an atlas spec by id or alias ([find_atlas_spec()]), and * dispatch to the correct loader with a single entry point ([get_atlas()]).
Loaders are referenced by name (character string) rather than by function so that registering a new atlas doesn't require the loader to already be exported at package load time.