Add complex<dual<T,N>> Eigen support, fix rpart bug

Michael Teschrequested to merge
feature/multidual-eigen-complex into master

Summary

  • Add matrix-level rpart/dpart/dconj functors for complex<dual<T,N>> in multidual_eigen.
  • Fix rpart(complex<dual<T,N>>) — was returning dual<T,N> (x.real()), now correctly returns complex<T> matching the original dual header.
  • Add 4 Eigen integration tests: rpart/dpart for dual and complex-dual matrices, scalar promotion, matrix multiply.
  • Bump version to 0.8.2.

Test plan

  • 450/450 tests pass (4 new Eigen integration tests)
  • expm benchmarks build and run with both old and new targets

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