Fix scaling mode in IC generators and std_one validation

[Ceyron]

This PR is partially breaking

• Fix build_scaling_array mode in IC generators: GaussianRandomField and RandomTruncatedFourierSeries used mode="coef_extraction" when pre-scaling Fourier coefficients before ifft. This mode doubles along every axis for non-zero, non-Nyquist modes, but irfftn only restores conjugates along the right-most axis. The result is that physical amplitudes on non-right-most axes are halved in 2D+. Changed to mode="reconstruction", which only doubles along the right-most axis. No behavior change in 1D (the two modes are identical when there is only one axis).

• Fix inverted std_one validation: The condition offset_range == (0.0, 0.0) and std_one incorrectly rejected zero offset with std_one=True (valid) and allowed non-zero offset with std_one=True (invalid). Flipped to !=.

Closes #34

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (691d050) to head (47a3baa).
⚠️ Report is 2 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
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+ Coverage   91.60%   91.61%   +0.01%     
==========================================
  Files          72       72              
  Lines        2560     2564       +4     
==========================================
+ Hits         2345     2349       +4     
  Misses        215      215              

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