Messages Need a Protocol Before They Need a Chat UI

A kiosk messaging system should define message state and delivery policy in the service layer before adding a chat UI.

BidirectionalMessagingService should own the message lifecycle, delivery rules, acknowledgements, read receipts, and cleanup.

Channel selection should be policy-driven, with SMS fallback when needed.

Centralizing these rules reduces inconsistent behavior across screens, closes audit gaps, and improves patient communication on shared devices.