Added XORoshiro-256 PRNG implementation paper and README with detailed behaviour

[Knogle]

Ahoy,

This one i forgot in the first PR, but i consider it to be important, as XORoshiro-256 is a heavily modified and not-yet existing adaption of XORoshiro-128**.

Added detailed documentation for the XORoshiro-256 PRNG implementation, including a comprehensive README and a formal paper outlining the mathematical and statistical analysis in the src/xor folder.

The README explains key differences between XORoshiro-256 and the original XORoshiro-128**, its optimization for nwipe, and usage recommendations. The accompanying paper provides an in-depth exploration of entropy propagation, performance considerations, and empirical test results.

This update formalizes the implementation and supports its use in non-cryptographic applications with high throughput requirements on 64-bit systems.

[Knogle]

Ahoy. I think this one will be the option to go for, in the near future.
As Xoroshiro algorithms can be massively accelerated through FMA instructions (this one isn't yet) I was able to get 4GB/s of throughput. So extremely suitable for massive amount of drives, I will make a PR soon.