Choose a more descriptive parameter name in place of `v` in `check_valid_ref_id()`

https://github.com/DiamondLightSource/httomo/blob/ae5610a83a691436e0e7deff3a71c2af3f89568e/httomo/ui_layer.py#L239-L252

Read the linked section of `httomo/ui_layer.py` around lines 239–252 and inspect `check_valid_ref_id()`. Determine what the parameter `v` represents from its use in that function, then choose a more descriptive name and update its references consistently. Done means the function uses that name without changing its behavior.

Written by the indexing model from the issue text.