9 cases: null/empty/malformed email -> BadRequest; null or
sub-8-char password -> BadRequest; duplicate email -> DomainException;
valid signup returns User with hashed password and isAdmin=false;
user is findable by email afterwards; EmailAddress vo lowercases
the domain. fails red - SignupUser class not yet defined.
7 cases: rejects spaces, double-@, empty input; trims whitespace;
lowercases domain only (preserving local-part case); equality by
normalized value; __toString and getDomain. fails red - class
App\\Shared\\ValueObject\\EmailAddress not yet defined.
