The private key is now the BIP85 HEX application output for 32 bytes at
m/83696968'/128169'/32'/{index}' from the BIP32 master key of the BIP39
seed, via sneak.berlin/go/secret/pkg/bip85, so any BIP85 tool reproduces
it and one mnemonic backs many keypairs through Params.Index. Tests
check the mnemonic-to-master step and the HEX application against the
BIP85 specification vectors and pin the resulting keypair.