CDN-195: The Switch Freezes

CDN-195 freezes the operator-only switch rule left open by CDN-194. The rule has no fitted threshold: choose the phase-gradient channel exactly when the driven response peak crosses the phase-gradient prediction; otherwise choose the source channel. With the inherited source selector, the switched rule hits all three existing floor anchors with maximum error 0.002976; with rounded sigma*k = 2/3, it still hits all three with maximum error 0.003024. This freezes a candidate switch, but it remains in-family rather than a new holdout, so D9 is not yet closed.