mix mailglass.stability.check (Mailglass v1.0.0)

Ensures that Swoosh.Email.t() types do not leak into the Mailglass public namespace, preventing downstream developers from accidentally coupling to our internal engine's types.

Exemptions (escape hatches and internals):

• Mailglass.Message.update_swoosh/2 (official escape hatch)
• Mailglass.Message.new/2 (deprecated compatibility path during the 1.x upgrade window)
• Mailglass.Outbound.send/2 (legacy compatibility bridge; deliver/2 is canonical)
• Mailglass.Compliance (internal utility)
• Mailglass.Adapters.* (internal implementations)