Artifact stale-guard reads now retry once on transient failure

The "stale guard" read that checks whether a hand-off artifact is out of date now recognizes transient, retryable failures, distinct from permanent errors, "gone" errors, other-organization errors, or never-published/public-unasked cases, based on either a retryable HTTP status or a transient-failure flag. When such a failure occurs, the read is now retried once after a fixed delay, and the retry is recorded in its telemetry.