Example: Perfect read-chain

Start with a designated $|+⟩$ root and make $c-1$ perfect copies. The state is

For an interface containing $m<c$ class members, every effect has zero bias. At $m=c$,

The projector onto $|{\mathrm{GHZ}}_c⟩$ has coherent probability one and dephased probability $1/2$. Thus the perfect-copy transition is a hard step at the full budget. This is an example at every chain length, not a universal statement that every budget-complete interface has maximal discrepancy.