Backend composition

Backend composition — contract and behavior

• Composition is explicit startup configuration; a container image does not auto-discover Apple hosts or carrier credentials.
• The router chooses among only registered backends that advertise the requested service and capability.
• mac-relay, direct-apple, iphone-jb, SMS, and RCS retain their own lifecycle and credentials behind the common Backend protocol.
• Changing composition does not rewrite existing chat affinity; migrations and operational cutover handle durable state.