ReadonlylengthPerceived length (see the module doc); what a paced flight divides by its speed.
The pose at path parameter s ∈ [0, 1] (time already eased by the caller).
OptionalatOptional own timing: the pose at normalised TIME t, given the easing. A
via uses it to ease each leg separately (rest at the intermediate pose).
A pre-computed interpolation between two poses.