ENH: Linked files export to one IFC

### Enhancement Description

Hello there,

is there a possibility to improve options of exporting Linked Revit files into one file?

Right now if user select one of "merging options":
- Export in same IFCProject
- Export in same IFCSite

...it exports the same Project/Site, but it generates new GUID for IfcSite (in case IfcProject "merge" was selected) and the same applies for IfcBuilding/IfcBuildingStorey and even for Built Elements in both scenarios.

Same IfcProject

Same IfcSite

As you can see in attached screens both files have same Project Information parameter values in Revit files:
IfcProject GUID
IfcSite GUID
IfcBuilding GUID

... yet it is not kept in that option

As an extra you can see that if one of "Merging options" is selected then even GUID of Wall in linked file (2wQW2ntCr23RvBvLXzel96) is changed in export.

Only when separate files are exported (either from source file or individually) then the correct value is kept in IfcProject/Site/Building/Level/Element/...

Is that possible to keep GUIDs of original file when trying to merge Links into one IFC export?

### Revit Version

2026.0.x

### IFC for Revit Addon Version

26.1.0

### Windows Version

11 24H2

Start by reproducing linked-file exports with “Export in same IFCProject” and “Export in same IFCSite” using Revit 2026.0.x and IFC for Revit 26.1.0, then compare the GUIDs against separate exports and the values shown in the issue. Done means the merged IFC preserves the original project, site, building, level, and built-element GUIDs where the source files share them.

Written by the indexing model from the issue text.