Admissibility of spacetime Fourier cutoffs

For a test, each spatial Fourier cutoff remains a smooth spacetime test with the same periodicity and terminal support.

theorem AVenhance.Infra.Parabolic.FourierGalerkin.TimeDependentTestAdmissibility.spatialFourierCutoff_zero (N : ℕ) : Section5.testFourierCutoff (fun (x : Homogenization.Vec 2) => 0) N = fun (x : Homogenization.Vec 2) => 0