[REQUEST] SDF Xor operation

Currently OpenVDB supports the following SDF operations:

SDF Union:
`min(d1, d2)`

SDF Intersection:
`max(d1, d2)`

SDF Difference (usually named SDF Subtraction):
`max(-d1, d2)`

However, it doesn't support the [XOR operation described by Inigo Quilez](https://iquilezles.org/articles/sdfxor/):
`max(min(d1, d2), -max(d1, d2))`

This operation finds regions where the sign mismatches.


It would be nice to include all standard SDF operations in OpenVDB.

Search the repository for the existing SDF union, intersection, and difference operations and their tests to find the relevant entry points. Add the requested XOR operation using the formula in the issue, then extend the nearby test coverage to verify the sign-mismatch behavior.

Written by the indexing model from the issue text.