Serialize Unity hinge reference with invariant culture

[sylvesterkaczmarek]

Summary

Make Unity-generated hinge joint ref values locale-independent.

MjHingeJoint.OnGenerateMjcf already uses MjEngineTool.MakeLocaleInvariant for range, but formats ref with the current process culture. On locales with a comma decimal separator, for example fr-FR, a reference value such as 1.5 can therefore be written as 1,5, which is not the intended MJCF numeric representation.

This change:

• serializes hinge ref through MjEngineTool.MakeLocaleInvariant
• adds a regression test that switches the current culture to fr-FR and verifies the generated value remains 1.5
• does not change parsing or runtime joint behaviour

Validation

The branch is based directly on current main, is one commit ahead, and changes only MjHingeJoint.cs and its existing test file.

The Unity EditMode suite was not executed in this environment, so no local Unity test pass is claimed.