According to Claude, the reason the CarpetX derivs test fail on Stampede3 is bad masking logic that fails for 512.

Here is Claude's proposed fix.

https://github.com/EinsteinToolkit/CarpetX/tree/bug/vec-fix

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2964/test-derivs-fails-on-stampede-with-nans