🎾 Intermediate-Axis Flip Lab: Euler's Equations, Polhodes and the Tennis-Racket Effect — 2D Polhode Map, Flip Delay and Stability

2D companion of the intermediate-axis flip lab: a body view with fixed angular momentum, the polhode portrait, angular-velocity traces, the logarithmic flip-delay law and conserved quantities, using the same Euler-equation integrator as the 3D scene.