kit v0.5.3

Simplified check: download, hash, compare, report.

Removed in-code retry logic, diagnostic logging, and CDN caching
workarounds from the check phase. Transient download failures are
the CI layer's responsibility (retry: max: 2 in pipeline config).
Kit check should surface problems, not mask them.

Investigated on GCE: the checksum mismatches seen on storr could
not be reproduced with the same container image and reqwest config.
The issue is transient and infrastructure-specific.