Creates a deterministic deriver function for a seed/keyset pair.
Parameters
seed: Uint8Array
keysetId: string
Returns SecretAndBlindingFactorDeriver
Remarks
For deprecated BIP-32 derivation this caches the shared keyset parent node once, so each counter
costs three child derivations instead of a full path walk from the master. Constructing an HDKey
node computes its public key, so fewer nodes means fewer EC multiplications.
Creates a deterministic deriver function for a seed/keyset pair.