Introduce randomness source to the platform

We need external non-determinism to come from the platform. Otoh, LiteBox itself should itself contain a CSPRNG (possibly rand-chacha?) that is seeded from the non-determinism coming from the platform, rather than depending _entirely_ on cryptographic security of the platform's randomness provider.

No files, tests, or entry points are named. Start by locating LiteBox's platform randomness boundary and the existing randomness implementation, then determine how platform non-determinism should seed an in-LiteBox CSPRNG. Done means the platform supplies the seed material without LiteBox depending entirely on the platform provider's cryptographic security.

Written by the indexing model from the issue text.