Another round of AI-assisted manual edits

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Free-form text area

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

• McStas: Polarisation appendix converted to new chapter
• McXtrace: Manual sections (e.g. running.tex) brought on par with McStas

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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:

Claude was given a draft manuscript (skeleton) of future McStas pol paper and asked to

• Upgrade the Pol appendix to a chapter with this structure
• Asked to include descriptions of precession algorithm and other pol-lib bits
• Asked to ensure parameter agreement between components and manual

(The McXtrace stuff is a "bonus", edits left behind from pre-3.8.4 release)

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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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Direct links to the McStas manuals on the PR branch here for convenience
https://github.com/mccode-dev/McCode/tree/pol-chapter-rework/docpkg/manuals/mcstas