mix mailglass.stability.check (Mailglass v0.3.2)

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 v0.1 API)
• Mailglass.Outbound.send/2 (deprecated v0.1 API)
• Mailglass.Compliance (internal utility)
• Mailglass.Adapters.* (internal implementations)