pub fn projected_kernel_matrix(
xs: &[Vec<f32>],
cfg: &ProjectedKernelConfig,
) -> QuantumResult<Vec<Vec<f32>>>Expand description
Build the full PQK Gram matrix K[i][j] = k_PQK(xs[i], xs[j]).
Features are computed once per data point and cached, so the cost is
O(m · embed) + O(m²·3n) rather than O(m²·embed).
§Errors
Returns QuantumError::EmptyInput for an empty data set; propagates
embedding errors.