Fix frequency array alignment in FFT shear calculation

The kx and ky frequency arrays were misaligned with the FFT output. The code
was creating frequency arrays with fftfreq(n+1)[1:] which skips the DC
component, but the FFT includes all frequencies. This caused incorrect
mapping between frequencies and FFT coefficients, leading to wrong shear
perturbations.

Fixed by using fftfreq(n) which properly matches the FFT output dimensions
and includes the DC component at the correct index.

Co-authored-by: Sierd <14054272+Sierd@users.noreply.github.com>

Author: Copilot

aeolis/shear.py

@@ -544,8 +544,8 @@ def compute_shear(self, u0, nfilter=(1., 2.)):
             return
 
         ny, nx = gc['z'].shape
-        kx, ky = np.meshgrid(2. * np.pi * np.fft.fftfreq(nx+1, gc['dx'])[1:],
-                             2. * np.pi * np.fft.fftfreq(ny+1, gc['dy'])[1:])
+        kx, ky = np.meshgrid(2. * np.pi * np.fft.fftfreq(nx, gc['dx']),
+                             2. * np.pi * np.fft.fftfreq(ny, gc['dy']))
 
         hs = np.fft.fft2(gc['z'])
         hs = self.filter_highfrequenies(kx, ky, hs, nfilter)