Artifact: PR Description — Carrier Adapter Retry-with-Backoff

Retry-with-backoff PR description -- what/why/how-verified/where-to-look -- exercises co-09.

What: adds exponential backoff (3 retries, 200ms/400ms/800ms) to CarrierAdapter.get_status(), the function that calls ParcelLink's /tracking endpoint.

Why: ParcelLink's API returns transient 503s under their own load spikes (roughly 2% of calls during their peak hours, per their own status page's historical incident log). Today those 503s propagate straight to the customer-facing shipment-status page as an error; a short retry absorbs almost all of them.

How verified: added a test that mocks ParcelLink returning two 503s then a 200, confirming the adapter retries and eventually returns the successful result; ran the existing adapter test suite unchanged (all passing).

Where to look first: carrier_adapter/retry.py, the backoff-and-retry loop itself -- everything else in the diff is unchanged call sites threading the new retries=3 parameter through.