SPI: Diagram

Use [excalidraw](https://excalidraw.com/) to to demonstrate how the system will work.
Each transaction includes different messages, so we must have a way to identify what each packet would be, I believe we can do this with its idepotency key

No files, tests, or entry points are identified. Start by mapping the system flow and the different messages in each transaction, then use Excalidraw to show how packets are identified, including the proposed idempotency key; done means the system behavior is represented clearly in a diagram.

Written by the indexing model from the issue text.