shouldAnnounce

Whether a receipt at txId is not-yet-announced for this (network, address).

• lastAnnouncedTxId == null (nothing announced yet) → announce. The upstream baseline already suppressed pre-existing history, so the first event that reaches here is a genuine new receipt.

• txId > lastAnnouncedTxId → a newer receipt → announce.

• otherwise → already announced (a replay or the other path beat us) → skip.