[Feature] Faster hologram reconstruction

• Added ReconstructionPlan, which precomputes once which part of a hologram’s spectrum ends up in the output window, and then reconstructs each hologram with a real-to-complex FFT, transforming only the needed columns of the spectrum and gathering the window directly, without full-size roll, fftshift or scaling passes. Build it with from_params() and reuse it for a whole stack. Reconstructing the wave and the brightfield image of a 4096x4096 hologram at 1024x1024 takes about a sixth of the time it did.

• reconstruct_frame(), reconstruct_bf() and HologramFFT are now thin wrappers around the plan, with the same signatures; results agree with the previous implementation up to floating point rounding. reconstruct_bf() gained the precision parameter. The index arrays of the plan are cached per geometry and array module, on the GPU for cupy, so calling the wrappers once per frame does not transfer anything per call. An out_shape larger than the hologram, or a slice_fft that is not the centered slice of get_slice_fft(), now raise ValueError instead of producing wrapped-around results.

• The precision parameter now selects the computation dtype explicitly: True transforms the hologram in float64 (complex128 results), False in float32 (complex64 results). Previously, float32 holograms were transformed in float32 regardless, and precision=False still returned complex128.

• numpy >= 2.0 is now required.