Compute a 64-bit dHash of a base64-encoded PNG, returned as a 16-char hex
string. Returns '' when the input cannot be decoded (callers treat an empty
hash as "no comparison possible" and fall back to their prior behavior).
Never throws — this is telemetry-grade and must not disrupt a run.
Compute a 64-bit dHash of a base64-encoded PNG, returned as a 16-char hex string. Returns '' when the input cannot be decoded (callers treat an empty hash as "no comparison possible" and fall back to their prior behavior). Never throws — this is telemetry-grade and must not disrupt a run.