Metric Markov cotype of ℓ₁ and Hilbert-space Lipschitz extension

Proves that real ℓ₁ has metric Markov cotype two, answering Mendel and Naor's question. Consequently, every Lipschitz map from an arbitrary subset of a real Hilbert space into ℓ₁ extends to the whole space with a universal multiplicative loss in its Lipschitz constant, resolving Ball's extension problem for this target.