Cannot change a Page name after instantiation

I wanted to set the page name, and right now TreeDiagram instantiates its Page without name so it's a change after the fact.

I can set page.name but it doesn't reflect in the generated drawio, because it actually needs to be set in the diagram.

The logical thing is to create a property with setter that reflects the diagram, similar to a lot of other properties around the library.

I have a branch already with the fix + new test.

Start at TreeDiagram, where the Page is instantiated without a name, and compare its name handling with the library's other properties. Verify that changing page.name is reflected in the generated Draw.io output, then run the new test mentioned in the issue to confirm the behavior.

Written by the indexing model from the issue text.