feat: Add FFT, DCT, and window functions - #17

Summary

This PR introduces signal-processing functionality to NDArray: complex and real FFTs, multi-dimensional transforms, discrete cosine transforms (types I–IV) with SciPy-style normalization options, and a library of window generators.

Motivation and context

Scientific computing workflows in PHP need the same foundational building blocks common in NumPy/SciPy: frequency-domain transforms with well-defined normalization conventions, robust inverse pairings, and standard tapering windows for spectral analysis and filtering-style pipelines. This PR adds those capabilities in a performance-oriented way (Rust + FFI) while keeping the PHP API explicit and testable.

What’s changed

• Added complex FFT/IFFT (1D and N‑D) and 2D helpers.
• Added real FFT / inverse real FFT with Hermitian-spectrum → real output behavior.
• Added DCT I–IV and inverse DCT with correct type pairing, including 1D, 2D, and N‑D variants.
• Added static window factories on NDArray (Bartlett, Blackman, Bohman,ming, Hanning/Lanczos, Triangular, Kaiser) with a symmetric/periodic option where applicable.
• Added new tests and documentation to cover the new functionality.

Breaking changes

None.