Contabo answers 429 to PUT bursts; at 16 concurrent uploads botocore's
default 4 retries ran out and the run died at tile 54/390. Adaptive retry
mode (client-side rate limiting, 10 attempts), a halved upload pool, and
per-tile failure isolation (a failed tile logs and is retried next run —
it never reached the manifest) keep the run alive through throttling.
