update bdy imp3 by hand, remove j and update blocking loop

[MetBenjaminWent]

PR Summary

Remove the j loop which is adversely affecting performance in the boundary layer, and improve blocking loops.

Performance data can be found in the umbrella issue, #106 (TBD)
closes #314

Reviewer note, I'm yet to performance test this one, as fixes included in #231 are blocking this, as is the lack of max time in Vernier, which will need developing.
I'm going to finish all of the Wave 2 ones noted in #106 together and get updated performance data for each of these issues, and progress in the boundary layer.

Could the change-set be reviewed sans the performance data, just to keep this ticket moving, but hold the final approval until I've added this? Thanks

Sci/Tech Reviewer:
Code Reviewer: @mo-alistairp

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
• All automated checks in the CI pipeline have completed successfully

Testing

• I have tested this change locally, using the LFRic Apps rose-stem suite
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
• I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
• Any new tests have been assigned an appropriate amount of compute resource and have been allocated to an appropriate testing group (i.e. the developer tests are for jobs which use a small amount of compute resource and complete in a matter of minutes)

trac.log

Test Suite Results - lfric_apps - bdy_imp3-by-hand/run15

Suite Information

Item	Value
Suite Name	bdy_imp3-by-hand/run15
Suite User	benjamin.went
Workflow Start	2026-03-05T14:47:32
Groups Run	ex1a_omp_developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.1	True
jules	MetOffice/jules@2026.03.1	True
lfric_apps	MetBenjaminWent/lfric_apps@bdy_imp3-by-hand	False
lfric_core	MetOffice/lfric_core@2026.03.1	True
moci	MetOffice/moci@2026.03.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.1	True
socrates	MetOffice/socrates@2026.03.1	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.1	True
ukca	MetOffice/ukca@2026.03.1	True

Task Information

✅ succeeded tasks - 51

Test Suite Results - lfric_apps - bdy_imp3-by-hand/run16

Suite Information

Item	Value
Suite Name	bdy_imp3-by-hand/run16
Suite User	benjamin.went
Workflow Start	2026-03-05T16:50:37
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.1	True
jules	MetOffice/jules@2026.03.1	True
lfric_apps	MetBenjaminWent/lfric_apps@bdy_imp3-by-hand	False
lfric_core	MetOffice/lfric_core@2026.03.1	True
moci	MetOffice/moci@2026.03.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.1	True
socrates	MetOffice/socrates@2026.03.1	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.1	True
ukca	MetOffice/ukca@2026.03.1	True

Task Information

❌ failed tasks - 2

Task	State
run_gungho_model_robert-moist-smag-BiP100x8-10x10_azspice_gnu_fast-debug-64bit	failed
run_gungho_model_straka_200m-alt1-BiP256x4-200x200_azspice_gnu_fast-debug-64bit	failed

• Errors not related to changes, timeouts which exist normally on trunk

✅ succeeded tasks - 1156

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

Performance Impact

• Performance of the code has been considered and, if applicable, suitable performance measurements have been conducted

AI Assistance and Attribution

• Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

Documentation

• Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

PSyclone Approval

• If you have edited any PSyclone-related code (e.g. PSyKAl-lite, Kernel interface, optimisation scripts, LFRic data structure code) then please contact the TCD Team

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
• Related Issues have been properly linked and addressed
• CLA compliance has been confirmed
• Code quality standards have been met
• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable