Add GH_SCALAR_ARRAY to argument_mod

[mo-alistairp]

PR Summary

Sci/Tech Reviewer: @TeranIvy
Code Reviewer: @MetBenjaminWent

This pull request is necessary to add the GH_SCALAR_ARRAY metadata type into LFRic to support arrays of scalars in LFRic. This PR goes alongside the corresponding PSyclone implentation which adds this support (both stfc/PSyclone#1312 and stfc/PSyclone#3101).

Code Quality Checklist

(Some checks are automatically carried out via the CI pipeline)

• 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

Testing

• I have tested this change locally, using the LFRic Core rose-stem suite
• If required (e.g. API changes) I have also run the LFRic Apps test suite
  using this branch
• If any tests fail (rose-stem or CI) the reason is understood and
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trac.log

Test Suite Results - lfric_core - gh_scalar_array_arg_mod/run1

Suite Information

Item	Value
Suite Name	gh_scalar_array_arg_mod/run1
Suite User	alistair.pirrie
Workflow Start	2026-01-19T10:24:41
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	mo-alistairp/lfric_core@add_gh_scalar_array_to_argument_mod	False
SimSys_Scripts	MetOffice/[email protected]	True

Task Information

✅ succeeded tasks - 372

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Performance Impact

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AI Assistance and Attribution

• Some of the content of this change has been produced with the assistance
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  (including attribution labels)

Documentation

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  confirmed that it builds correctly

PSyclone Approval

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  interface, optimisation scripts, LFRic data structure code) then please
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  [email protected]

Sci/Tech Review

• I understand this area of code and the changes being added
• The proposed changes correspond to the pull request description
• Documentation is sufficient (do documentation papers need updating)
• 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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• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable

[mo-alistairp]

Hi @TeranIvy, this should now be ready for review since the equivalent PSyclone change has been merged

[TeranIvy]

Nice work, @mo-alistairp! I would suggest a minor change in metadata reorder, please see inline.

I tested the fork, and all is fine.

[TeranIvy]

Retested the updated fork after the review change, and all is fine. Passing to @MetBenjaminWent for code review.