params: g=1.0 V=4.25 K=3.0 n=5
bistable window J in [0.548, 1.508]  (width 0.960)
max stable-state count = 2  (2 => true bistability)
up-tip at J≈1.508 ; down-tip at J≈0.548 ; gap=0.96  => hysteresis
linear/PK: x*=J/g single state for all J — no fold, no tipping.
