getBlockHashAtHeight

Block hash at height — the chain-reset checkpoint lookup. FAILURE-TOLERANT: any query error (network, GraphQL, parse) is BlockHashLookup.Unavailable rather than a throw, so the reset check SKIPS on a transient hiccup instead of risking a wipe of a healthy wallet. A VALID response whose block field is null (the chain is shorter than the checkpoint) is BlockHashLookup.NotOnChain — kept distinct from Unavailable so the guard can treat the two opposite meanings differently. CancellationException is rethrown so cancellation isn't swallowed.