Implement managed ForwardMultidim using separable 1D FFT

[bryancostanich]

Summary

Implements ManagedFourierTransformProvider.ForwardMultidim and BackwardMultidim using separable 1D FFT decomposition (process each dimension independently using existing 1D FFT).

Previously these methods threw NotSupportedException, making Fourier.Forward2D() and Fourier.Inverse2D() unusable without a native provider (MKL).

Motivation

On platforms where MKL is unavailable (macOS ARM64, other non-x86 targets), multi-dimensional FFT was completely broken. The fix is straightforward since the 1D managed FFT already works correctly.

Implementation

• For each dimension of the N-D array, extract 1D slices along that dimension, apply the existing 1D FFT, and write results back
• Handles both power-of-two (Radix-2) and non-power-of-two (Bluestein) dimension sizes
• Scaling is applied once after all dimensions are processed, matching MKL provider behavior
• Works for arbitrary N-D arrays, not just 2D

Tests

Added four new tests to FourierTest.cs:

• Forward2DAndInverse2DRoundtrip64 -- Complex64 roundtrip with random data (4x8)
• Forward2DAndInverse2DRoundtrip32 -- Complex32 roundtrip with random data (4x8)
• Forward2DKnownDCValue64 -- Constant array produces energy only in DC bin (4x4)
• Forward2DNonPowerOfTwoDimensions64 -- Non-power-of-two dimensions (3x5) exercise Bluestein path

All 87 FFT tests pass on both net8.0 and net48.

Fixes #1150