Design and review discussions have surfaced diverging views of how the
chat stack is layered. Add a normative reference for the target
architecture so those discussions share one picture.
- Defines the layers — Conversation Core, Client (Generic + Platform),
Application — with a responsibilities table and call/event flow.
- States the governing principles: caller-driven sync core, sync results
vs async observations, a data-not-behavioural boundary, and
easy-edge/powerful-core.
- Documents runtime independence (Generic Client optional; runtime choice
at the Platform Client) and the Services/ConversationTypes axis.
- Aligns terminology with the chat specifications.
