Well-define default controller trajectory calculation logic when lane following

Describe the feature

As an OpenSCENARIO 1.x scenario editor and scenario runtime implementer (@AlexHeavensFive), I want a clear, concise and precise definition of how the OpenSCENARIO 1.x "default controller" should construct its trajectory when lane following for all relevant situations so that the scenarios we create are portable between OpenSCENARIO 1.x tooling and we avoid data migrations in future necessary to make us so.

Relevant situations:

• Simple lane following
• Lane merge
• Lane split
• Junctions
• ...?

Describe the solution you would like

A clear, concise and precise definition of how the OpenSCENARIO 1.x "default controller" should construct its trajectory when lane following for all relevant situations. Likely in the user guide.

The agreed solution should be a good balance of the idealised trajectory definition, but also consider what the landscape of OpenSCENARIO tooling currently do.

Describe alternatives you have considered

It could be argued that trajectory control is too granular a level for OpenSCENARIO to be concerned with.

Describe the backwards compatibility

Potentially very backwards incompatible as existing tools are possible highly divergent. This will only get worse the longer we don't address it.

Additional context

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