fix(fft): handle len==1 input without panicking#272
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fft() states its precondition only via ensure!(v.len().is_power_of_two()), and 1usize.is_power_of_two() == true. The implementation, however, unconditionally writes to twiddle[1], even though twiddle has length 1 when v.len() == 1, causing an out-of-bounds panic. Before reaching that point, bitreverse_permutation also computes a right-shift of usize::BITS - length.ilog2() bits, which equals a full-width shift for length == 1 and panics under overflow-checks. Both failures share the same root cause: the n == 1 case was never handled by the general radix-2 code path, which assumes n >= 2. A length-1 FFT is mathematically the identity transform, so handling this case with an early return preserves correctness while avoiding the panic. Reachability: all current call sites of fft() in zkml/src/layers/convolution/mod.rs construct their input length as 2 * n_x * n_x or 2 * filter_size(shape), which is structurally >= 2. So this was a latent API defect, not reachable through any existing call path, but it violates the function's documented precondition and would panic for any future or external caller passing a single-element vector. Adds minimal PoC regression tests (fft_single_element_should_not_panic, ifft_single_element_should_not_panic) that reproduce the panic before this fix and pass after it.
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Handle len == 1 in fft() with an early return to avoid a panic caused by the radix-2 implementation assuming n >= 2. Add regression tests for both FFT and inverse FFT