Is the walk-run transition Froude number the kinematic ceiling, or a biological constant?

The compute is trivial; the work is literature assembly, which is why this is still a draft. Per species at the transition: speed, leg length under one consistent definition, and stride length. Froude is linear in leg length, so a 20% definitional difference moves it by 20% and would swamp the effect.

The contact-loss ceiling in average-speed units is about 0.62 at observed gait geometry, not the textbook 1.0. Animals transition near 0.45, so roughly three-quarters of the ceiling.

Branch 1 predicts the points lie on the ceiling curve — species with longer relative strides transitioning at lower Froude. Branch 2 predicts a flat scatter. Branch 3 predicts a line parallel to the ceiling at a constant fraction.

Prior: relative stride length at the transition clusters near 2.0 across species, and a quantity that does not vary cannot explain one that does.