Artifact: Design Doc Open Questions — Shipment ETA Recalculation

ETA-recalculation design doc, decided and undecided items kept explicitly separate -- exercises co-04.

Decision: recalculation runs synchronously on every shipment status update -- not on a schedule -- because ETA staleness between updates is the exact problem this doc exists to fix.

Open Questions:

• What's the fallback if ParcelLink's own ETA field is missing for a shipment? (undecided -- proposal: fall back to a static origin-to-destination average, but this needs the data team's input on whether that average is even tracked today.)