Fix FFT failure for array sizes 2^21 and 2^22 #2997
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Fixes ml-explore#1800 The FFT was failing for arrays of size 2^21 and 2^22 with a "kernel not found" error because the four-step decomposition was creating sub-FFTs (n1=2048 or 4096) that exceeded Metal's threadgroup memory limit. This fix lowers the four-step FFT threshold from 4096 to 1024, forcing recursive decomposition earlier and ensuring all constituent FFTs fit within Metal's 32KB threadgroup memory limit. Changes: - Added MAX_SAFE_FFT_SIZE constant (1024) - Updated plan_fft to use MAX_SAFE_FFT_SIZE instead of MAX_STOCKHAM_FFT_SIZE - Added test case for 2^21, 2^22, 2^23 to prevent regression
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Fixes #1800 by lowering the four-step FFT threshold from 4096 to 1024, forcing recursive decomposition to avoid Metal threadgroup memory limits. Added test case for 2^21, 2^22, 2^23.