PowderN fix potential edge cases

[willend]

Free-form text area

Please describe what your PR is adding in terms of features or bugfixes:

Patches for 2 rare, potential pitfalls:

1. Over/underrun of reflection list in calc_xsect()
2. Add guard for rand01()==1.0 edge case

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Declaration of use of AI-tools

• Please add a checkmark here if you used AI-tools during the work for this contribution
• Furter, please describe how / where and for what the tools were used:

Once in a while one or more of the Test_Powders tests are off. Used Claude-prompts to pinpoint potential issue areas in PowderN.

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Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:

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PR Checklist for contributing to McStas/McXtrace

For a coherent and useful contribution to McStas/McXtrace, please fill in relevant parts of the checklist:

• My contribution contains something else

  • Explanation is added in free form text above or below the checklist

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[willend]

Put to 'draft' mode - it seems that the issues we attempted to fix here were rather corrected via #2634