Auto-Coupled Pleth Signal Loss at Low Perfusion

At very low PTT settings combined with very low Pleth Amplitude, real monitors lose the pleth signal entirely and display SpO2 unable to read. Simulating this signal-loss threshold is a meaningful teaching scenario. Proposed behavior: when Pleth Amplitude falls below ~10% AND PTT is at or near maximum (450+ ms), the SpO2 channel should drop the pleth waveform, display SpO2 as ---, and trigger an SpO2 INOP alarm. This teaches students that peripheral SpO2 measurement becomes unreliable in severe shock or profound vasoconstriction. Deferred to v1.5 - in v1 the waveform always renders if amplitude > 0%.