int8 (signed=true, 8 bits) or uint8 (unsigned) indices, one element
per input element — NOT bit-packed here; call packBits separately
(mirrors the reference's turbo_quant_encode_key / turbo_quant_encode
split: quantize() returns unpacked indices, turbo_quant_encode packs
sub-8-bit ones).
int8 (signed=true, 8 bits) or uint8 (unsigned) indices, one element per input element — NOT bit-packed here; call packBits separately (mirrors the reference's turbo_quant_encode_key / turbo_quant_encode split: quantize() returns unpacked indices, turbo_quant_encode packs sub-8-bit ones).