The thermostat: a sub-spinodal step is corrected

The preoptic thermostat is the defended upper R19 basin at the setpoint. A sub-spinodal ambient step is corrected back toward the setpoint (1.1832 in R19 units); once the step exceeds the spinodal 0.6376 the euthermic basin disappears and the defense is overwhelmed. Error stays bounded while sub-threshold.

RT2. The thermostat corrects ambient steps below the R19 spinodal and fails abruptly past it -- a regulated comparator with a hard threshold, not a proportional knob. The setpoint (~37 °C) is cited [L]; the mechanism is [V]; the absolute gain and latency are [O].

Bounded correction below threshold

Stepping the ambient drive while holding the upper basin, every sub-spinodal step is pulled back to the setpoint with bounded error (True); steps beyond the spinodal collapse the basin (True). The spinodal 0.6376 is the comparator's authority limit.

Why a hard threshold matters

A real thermostat tolerates weather but is overwhelmed by extremes; the R19 spinodal gives exactly that shape -- robust regulation up to a sharp edge, then loss of control. The same edge reappears as the fever/hyperthermia boundary and the torpor switch.