Tensor saturation for even spin groups

Proves saturation factor one for $\mathop{\mathrm{Spin}}\nolimits (2n)$, n ≥ 2: for three dominant integral weights whose sum lies in the root lattice, an invariant at any common positive integral dilation already gives an invariant at the original weights. This resolves the type-D part of the simply-laced saturation conjecture.