osmaczko c59f608ee6
docs: add target architecture reference
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.
2026-06-09 08:41:05 +02:00
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