A carrier-only causal-break witness

Let $t\ge 1$, so $w=w_t\in (0,1)$. Freeze the scar register between two identical sensing intervals. Assume $0<C(T)\le 1/2$.

Prepare ${\rho }_{+y}$, sense for time $T$, and measure $P_0,P_1$. Immediately re-prepare the same ${\rho }_{+y}$, regardless of the outcome, and perform the same future sensing and node measurement.

The intervening complete causal-break instrument is

It acts only on $Q$. No scar-label measurement or external damage flag is used.