Function spatial_hadamard_distance_warning 

pub fn spatial_hadamard_distance_warning(
    graph: &BlockGraph,
) -> Option<&'static str>

Return the user-facing distance warning when graph uses a construction whose fixed-bulk circuit distance is not guaranteed to equal d.

This is the pre-flight form, for front ends that warn before compiling. A completed compilation reports the same advisory in CompileArtifacts::warnings, which cannot be forgotten.