PR: Can't overwrite *_Common.LoadBearing with a user defined parameter

### Problem Description

... Hello.
I know not all the families have the "Loadbearing" instance built in parameter.
Therefore we have had to implement an office-wide ARC-Loadbearing parameter which we also prefer to use as type parameter.
The problem is that when we use this parameter and export the IFC with a "supposedly" correct mapping file, the corresponding *common.LoadBearing parameter in the exported IFC does not react and takes only the value from the "Loadbearing" built in parameter.

Is there a way to overwrite it?
I think the translator works because, for test purposes, I assigned the ARC_Tragwerk (loadbearing in german) to other common parameters, and it worked.

Is perhaps my syntax wrong, or is it actually impossible?

```
Pset_WallCommon LoadBearing ARC_Tragwerk
Pset_WallCommon Combustible ARC_Tragwerk
Pset_WallCommon IsExternal ARC_Tragwerk
Pset_WallCommon Reference ARC_Tragwerk
```

Thanks in advance!

### Revit Version

2024.0.x

### IFC for Revit Addon Version

Other / None

### Windows Version

11 22H2

Start by reproducing the Revit 2024 export with the shown mapping entries for Pset_WallCommon, especially LoadBearing and ARC_Tragwerk. Trace how the exporter handles the built-in LoadBearing value versus mapped user-defined parameters, and define done as confirming whether the mapping can override it or documenting the supported behavior.

Written by the indexing model from the issue text.