In the context of the `@bsv/sdk`, what method is used by a master private key to derive a Type 42 child key for a specific invoice number and counterparty public key?

When Alice wants to derive a public key for Bob using Type 42 derivation, she uses Bob's public key and her own master private key. What is the method call on Bob's public key object in `@bsv/sdk`?

In the `BapMasterBackup` interface from the `bitcoin-backup` library, what specific field name indicates that the backup is in the modern Type 42 format rather than the legacy BIP32 format?

Unlike BIP32, Type 42 key derivation allows for 'unlimited invoice numbering space'. What data type is typically used for the 'invoice' or 'purpose' parameter in the `deriveChild` method?

What are the two key components combined to derive a shared child key in Type 42 derivation, ensuring that both parties can arrive at the same key universe without sharing master secrets?