Gateway for the optional MJML NIF dependency ({:mjml, "~> 5.3"}).

:mjml is the Hex package name (a Rust NIF binding to the mrml crate). The Elixir module it provides is Mjml — note the :mjml vs :mrml distinction called out in STACK.md. Used by Mailglass.TemplateEngine.MJML when adopters opt into MJML as an alternate rendering path (AUTHOR-05).

HEEx is the default Mailglass.TemplateEngine (D-18 project-level); MJML is strictly opt-in. When :mjml is absent and an adopter configures the MJML engine, Mailglass.Config.validate_at_boot!/0 raises %Mailglass.ConfigError{type: :optional_dep_missing}.