Brick #1: Typed participant roles let supply-chain handoffs record who did what

This article introduces Brick #1, an experimental proof in a series on verifiable supply chains, addressing the problem that handoff events record only addresses without revealing whether they belong to an organization, agent, or device. The approach adds optional typed participant roles (custodian, operator, device, sensor) to the existing handoff event payload, each with an address, one of seven types, and an optional label. These roles are self-declared by the party performing the handoff and are treated as advisory metadata, never used for identity, authorization, or payment checks. Roles are validated at write time, stored in the event, and surfaced per hop by the replay read model along with who asserted them. Events written before this change replay unchanged because all role fields are optional.