Function validate_scalar_connection 

pub fn validate_scalar_connection<I>(
    kind: ScalarConnectionKind,
    ports: &[ScalarPortContract<I>],
) -> Result<(), ScalarConnectionViolation>
where
    I: Eq,

Validate the current scalar Port compatibility algebra.

The check is independent of syntax ordering and graph storage. Signal causality follows Port direction: exactly one output and one or more inputs must share one dimension. Conserving markers share one dimension, while scalar physical Ports share one exact nominal identity.

Errors

Returns the first structural incompatibility in a stable rule order.