Improvements to coupled model testing

### Brief description

Following a meeting between GMED and SSD a proposal was put forward to improve the testing of the coupled model within rose-stem. The aim of this is to ensure that the testing within LFric development is representative of how the coupled model is used in GMED (and eventually operations). Ultimately, we want to get to the point of having coupled tests as follows:
- Compiled from lfric_atm in the same way as GMED workflows
- Match science developed by GMED and operational workflows
- Test restartability of the model
- Run in climate and NWP configurations

### Further details of the issue.

Below is a list of tasks to achieve the aims above. Issues for each of the tasks can be added to the table (and linked as sub issues)
| Task | Issue and/or Pull request | Owner | Milestone | Status |
|:-----|:----------------------------|:--------|:------------|---------|
| Add capability to compile from lfric_atm | #486 | Paul Earnshaw | [Summer 2026](../milestone/3) | On main |
| Fix coupled checkpointing in lfric_apps/core | #505 | Tim Graham | [Summer 2026](../milestone/3) | On main |
| Reduce resolution of lfric_coupled test to C48ORCA1 | #559 | Zoe Plain | [Autumn 2026](../milestone/4) | |
| Compile lfric_coupled rose stem test from lfric_atm | #624 | James Bruten | [Autumn 2026](../milestone/4) | On main |
| Write checkpoint, switch to XIOS2 & turn on stochastic physics | #774 | Ian Boutle | [Autumn 2026](../milestone/4) | in review |
| Add CRUN/NRUN testing to lfric_coupled test | #774 | Ian Boutle | [Autumn 2026](../milestone/4) | in review |
| Add an NWP configuration test (OSTIA lakes, no rivers) | | Tim? | Spring 27 | |
| Upgrade science of all coupled tests to GC6 science | #702 | Ian B | [Autumn 2026](../milestone/4) | On main |
| Add a weekly test at higher resolution | | Tim? | Spring 27 | |
| Add rose ana testing of diagnostics | #591 | SimIT | | |

Status Summary

- [x] **Add capability to compile from lfric_atm** #486 @pdearnshaw
- [x] **Fix coupled checkpointing in lfric_apps/core** #505 @timgraham-Met
- [ ] **Reduce resolution of lfric_coupled test to C48ORCA1** #559 @ZoePlain
- [x] **Compile lfric_coupled rose stem test from lfric_atm** #624 @james-bruten-mo
- [ ] **Write checkpoint, switch to XIOS2 & turn on stochastic physics** #774 @iboutle
- [ ] **Add CRUN/NRUN testing to lfric_coupled test** #774 @iboutle
- [ ] **Add an NWP configuration test (OSTIA lakes, no rivers)** # @
- [x] **Upgrade science of all coupled tests to GC6 science** #702 @iboutle
- [ ] **Add a weekly test at higher resolution** # @
- [ ] **Add rose ana testing of diagnostics** #591 @

#### A note on XIOS versions
Currently, coupled tests use XIOS2.5 whereas atmosphere only tests use XIOS2.0. Currently, this is set via variables in https://github.com/MetOffice/lfric_apps/blob/main/rose-stem/site/meto/common/suite_config_ex1a.cylc. There shouldn't be any need to have differences in *.xml files between these two versions.
**Edit: 10/9/2026: With apps 3.2 checkpoint/restarting doesn't work in coupled model with stochastic physics seeds and XIOS2.5. However, it does now work with XIOS2.0**

By the Autumn 2026 release, we should aim to move all configurations to XIOS3.0. This will be a breaking changes as xml files will need to be different for XIOS3.0 compared to XIOS2.0 xml files. The module will still differ for coupled tests because XIOS needs to be compiled with OASIS linked for coupled models (it would be really nice if we could run atmosphere only with an XIOS executable built with OASIS linked). Work is ongoing to ensure that coupled models can be run with XIOS3 (See https://github.com/MetOffice/tcd-XIOS3-extras/issues/57)

Start by reviewing the task table and the linked issues, then inspect rose-stem/site/meto/common/suite_config_ex1a.cylc and the existing coupled tests. The issue is an umbrella for several configuration, checkpointing, resolution, and diagnostics tasks, so completion requires selecting and scoping an unfinished subtask with its own acceptance criteria.

Written by the indexing model from the issue text.