Correct bug in global mesh

[mike-hobson]

PR Summary

Sci/Tech Reviewer: @ukmo-juan-castillo
Code Reviewer: @cameronbateman-mo

In global_mesh_mod there is a method (get_cell_id) that returns the cell id of a cell that is a given x- and y- distance from a starting cell id. A bug has been introduced into this routine so that if a negative x-direction distance is given, it inadvertently reverses the sense of the y-direction. This hasn't affected us so far, as our usage has only been with positive x-offsets.

A fix has been agreed with the author of the change that introduced the bug (#220) that simply reverses the sense of the y-direction for negative x-direction. This maintains the functionality that #220 added, but also fixes the broken behaviour.

I have updated the unit tests to add tests that would have picked up this problem and made them a little tidier

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
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Testing

• I have tested this change locally, using the LFRic Core rose-stem suite
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trac.log

Test Suite Results - lfric_core - fix_global_mesh_bug/run1

Suite Information

Item	Value
Suite Name	fix_global_mesh_bug/run1
Suite User	mike.hobson
Workflow Start	2026-03-12T11:13:52
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	mike-hobson/lfric_core@fix_global_mesh_bug	False
SimSys_Scripts	MetOffice/[email protected]	True

Task Information

✅ succeeded tasks - 384

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Performance Impact

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AI Assistance and Attribution

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Documentation

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PSyclone Approval

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Sci/Tech Review

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• Sufficient testing has been completed

(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

• All dependencies have been resolved
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• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable

[ukmo-juan-castillo]

The changes match the code author's proposed solution. New tests have been added to fully check the functionality of the relevant code and they pass successfully. I personally run other tests successfully. Everything is in order and the new tests are welcome.

[mo-rickywong]

This change Looks fine to me as code owner