B20 Memo Tracker

🆕 B20 adds memos to transfers/mints/burns (transferWithMemo) for payment IDs, compliance tags and settlement correlation — a field ERC-20 has no equivalent for. Reads a token's on-chain Memo event history, optionally filtered by a specific memo (bytes32) or the caller wallet. The settlement-reconciliation primitive for agents paying over B20 stablecoins.