Artifact: ADR-0001 — UTC Timestamps for Shipment Events

One-decision ADR, UTC timestamp storage -- exercises co-06.

ADR-0001: Store All Shipment Event Timestamps in UTC

Status: Accepted

Context: Shipment events originate from warehouses across three US time zones. Storing each event's timestamp in the warehouse's local time made cross-zone comparisons (e.g., "did this shipment leave before it arrived") unreliable -- a shipment briefly appeared to arrive before it shipped when compared across a time-zone boundary.

Decision: every shipment event's timestamp is stored and transmitted in UTC. Display-layer code (customer-facing SMS/email, the Warehouse Ops Portal) converts to local time only at render time, never before storage.

Consequences: all existing event-producing code paths need a one-time audit to confirm they already emit UTC (two did not, and were fixed as part of this change). Every future producer must follow the same rule -- documented here so a reviewer can point to this ADR in code review instead of re-litigating the question.