Muons (south): Active beam compensation

# Feature Description

As a user/instrument scientist of the south side muon instruments, I would like my beam to remain stable when other muon instruments change the power of their main magnets.

## Explanation
The main magnets on the south side muon instruments (particularity EMU) are strong enough to alter the beam position in other instruments. This can be corrected by applying linear scaling factors to the beam positioning magnets on those instruments based on the known strength of the magnet at a given point in time (the steering magnets are weak enough that their impact on other beam-lines in negligible).

MuSR can run in two modes, each of which will need to be adjusted for.

### Additional Information
This was[ first proposed](https://stfc365.sharepoint.com/sites/ISISProjects-Hub/Lists/Project%20List/All%20ISIS%20Projects%20%20Open.aspx?FilterFields1=ListID&FilterValues1=672%2E000000000000%3B%23816%2E000000000000&FilterTypes1=Number&viewid=2180a052%2D20c1%2D4f00%2D9018%2D48e893ad2986) in 2018. In 2019 a meeting took place and was [documented](https://isiscomputinggroup.github.io/ibex_developers_manual/specific_iocs/magnets/Muon-Active-Compensation.html). No further action was taken as it was easiest to implement this when all instruments were migrated to IBEX. With the migration of HiFi earlier this year, this has now been completed and so we are in a position to do this work. A meeting took place in September 2025 where it was agreed the requirements were broadly outlined in that documentation (which has been updated following that meeting).

_{See our [Git Workflow](https://isiscomputinggroup.github.io/ibex_developers_manual/processes/git_and_github/Git-workflow.html).}

# Acceptance Criteria

- [ ] An IOC exists that applies appropriate corrections to beam positions for each of the south side instruments based on the known strength of the other instruments main magnets.
- [ ] Information on this IOC is exposed to users
- [ ] [Existing documentation](https://isiscomputinggroup.github.io/ibex_developers_manual/specific_iocs/magnets/Muon-Active-Compensation.html) for what is planned is updated to be documentation for this IOC.
- [ ] Testing has taken place with the relevant muon beam-lines out of cycle to ensure it functions as expected

# How to Review
_{Before making a PR...}
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- _Provide screenshots of the feature to help the reviewer if relevant_

_{_If not applicable, write "Not applicable"_}

[comment]: <> (FILL IN BELOW)
> ...

_{**To the reviewer: Make sure to [update submodules](https://isiscomputinggroup.github.io/ibex_developers_manual/processes/git_and_github/Git-workflow.html)!**}

Discussed 00:48:50 - in planning 02/10/25 move to next sprint
DIscussed 08/01/2026 14:44