Honor Newton mimic enabled state

[Miko997]

Summary

Read newton:mimicEnabled when importing Newton mimic constraints instead of using the follower joint's physics:jointEnabled value. The deprecated MJC equality path keeps its existing behavior.

Wire the existing Newton token definition into the standalone USD decoder target and add a headless regression.

This addresses the mimicEnabled part of #3472. The other findings are outside this change.

Testing

• focused USD decoder regression
• 69 existing USD/MJCF tests
• full USD-enabled build
• 2,058 tests passed, 1 existing locale test skipped
• pre-commit hooks

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