:swoosh_subject deletes the email subject.
A direct subject(email, value) call is changed to subject(email, ""). In Swoosh.Email.new/1, the :subject option is removed, falling back to Swoosh's default empty subject.
Where the core :string family also runs, the direct-call replacement takes Mutare's overlap precedence: the string-literal mutants on that subject argument (empty and sentinel) are dropped, leaving this family's single note-carrying mutant. On the new/1 side the option removal and the core empty-string mutant coexist (they are minted at different nodes) and are behaviourally equivalent.