Verify a contiguous chain of headers: each header's prevBlock must equal the
previous header's hash, and each must satisfy its own embedded PoW target.
NOTE: this does NOT validate difficulty retargeting rules against consensus;
callers anchor trust via checkpoints / multi-source tip comparison instead.
Verify a contiguous chain of headers: each header's prevBlock must equal the previous header's hash, and each must satisfy its own embedded PoW target. NOTE: this does NOT validate difficulty retargeting rules against consensus; callers anchor trust via checkpoints / multi-source tip comparison instead.