Re: Seeking the correct term of art for the (unique) role that is usually called "postgres"—and the mental model that underlies it all

Nope, the name of the bootstrap user is the one supplied to initdb via the --username argument.  Period.  It need not match any name on the host operating system and it will still be the bootstrap superuser's role name.

Yes, the description for --username probably should be modified to read:

"Selects the user name of the cluster's bootstrap superuser."  Or just consider a "cluster superuser" the term d'art...since most people would just refer to any old role having superuser authorization as being plain ole "superuser".  The fact that is says "database superuser" is the same holdover effect as the fact that "init db" means "init database" even though it actually initializes a cluster.

David J.