Function est_dot_bit4 

pub fn est_dot_bit4(
    query: &Bit4Query,
    codes: &[u8],
    factors: Bit4Factors,
) -> Result<f32, QuantError>

Estimates a dot product directly from one packed four-bit row.

The native packed kernel scores unsigned nibbles directly, then applies dot(z, 2*u - 15) = 2*dot(z, u) - 15*sum(z) once per row. The query’s block-interleaved representation pairs directly with high- and low-nibble vectors, so scoring needs no expanded row, ZIP interleave, or per-vector grid conversion. If the prepared query reconstructs as scale*z, the integer kernel gives <y,q_hat> = scale * dot(2*y,z) / 2; the stored correction then yields the Extended-RaBitQ estimate.

For an odd dimension, the unused low nibble must be canonical zero padding.

Errors

Returns QuantError::CodeLength for a dimension mismatch and QuantError::NonZeroPadding for non-canonical trailing fields.