pub fn pack_nibbles(codes: &[i8]) -> Vec<u8> ⓘExpand description
Pack signed 4-bit codes ([-7, 7]) into bytes, two nibbles per byte,
low nibble first. The nibble is the two’s-complement low 4 bits, so
-7..=7 maps to 0x9..=0x7 and unpacks back exactly via sign extension.