//! `<tendon>` element AST. Only **fixed** tendons are represented — a fixed
//! tendon is a linear combination of joint coordinates, `L = Σ coefᵢ·qᵢ`, used
//! for actuator transmission and (via `<equality><tendon>`) length coupling.
//! Spatial tendons (site-routed cables with wrapping) are out of scope.
/// One `(joint, coefficient)` term of a [`FixedTendon`].
/// A `<tendon><fixed>` element: the scalar length `Σ coefᵢ·qᵢ` over its joints.