Reconsider the QR model stuff

Same kind of question above applies here. This was a very specific method to support deserialization on the `compas_xr_unity_assembly` [application](https://github.com/compas-dev/compas_xr_unity_assembly/blob/f1516ca568b101447507aebc28a594bdc358df3e/Assets/Scripts/DatabaseManager.cs#L817). _(because it was a bit easier in the application logic to deserialize as an assembly when we were doing all the manual deserialization)_ This is just a general comment. I think with the move to proto this should change kind of specific, but a good discussion point I think....😄

_Originally posted by @jckenny59 in https://github.com/compas-dev/compas_xr/pull/23#discussion_r3829994002_

Start with the linked DatabaseManager.cs location around line 817 and read the originating discussion in pull request #23. Compare that application-specific assembly deserialization with the proposed move to proto; done requires an agreed direction for the QR model and its deserialization approach.

Written by the indexing model from the issue text.