unregisteredNightUtxos

NIGHT UTXOs (from nightUtxosJson) that are NOT yet generating dust — i.e. still need a registration. Matched natively against the current synced dust state's backing-nights, so it's correct regardless of the cache-layer tokens. A null/absent dust state means nothing is registered yet, so all are returned. Drives MidnightSdk.registerForDustGeneration's per-UTXO registration loop.