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.
Remove KeyFromHex, Seal, and Open with their errors and constants. The
library only derives crypto_box keypairs from BIP39 mnemonics; callers
use golang.org/x/crypto/nacl/box directly for everything else.
sneak