[mypyc] Add librt.random module

[JukkaL]

The stdlib random module is fairly often used in performance critical code, and it's not super efficient. Add librt.random with a subset of the stdlib module interface that is optimized for performance when compiled.

Use ChaCha8 as the algorithm. Based on some research, this is a modern, high-quality PRNG algorithm. It's used by Go math/rand/v2, among others.

This is a non-cryptographic PRNG, similar to the stdlib random module (but this uses a different algorithm).

I used Claude Code and Codex to write all the code, but I iterated on it quite a lot and did a bunch of manual validation and code review. I also asked Codex to explicitly check that the ChaCha8 implementation is correct by comparing it to a reference implementation.

Use thread-local RNG state for module-level functions to enable good scaling in free-threaded builds. There's some extra complexity from having to free the state at thread exit.

I asked a LLM to generate and run a benchmark, and here are the results on 3.14:

  ┌─────────────┬─────────────────────┬────────────────────────┐
  │  Function   │ vs stdlib(compiled) │ vs stdlib(interpreted) │
  ├─────────────┼─────────────────────┼────────────────────────┤
  │ random()    │        3.2x         │          4.8x          │
  ├─────────────┼─────────────────────┼────────────────────────┤
  │ randint()   │        16.6x        │         18.0x          │
  ├─────────────┼─────────────────────┼────────────────────────┤
  │ randrange() │        14.5x        │         16.2x          │
  ├─────────────┼─────────────────────┼────────────────────────┤
  │ choice()    │        12.9x        │         10.3x          │
  └─────────────┴─────────────────────┴────────────────────────┘

choice() was replaced with randrange when using librt, since we don't provide it as part of this fairly minimal API.

[p-sawicki]

the _api files shouldn't be necessary when building the module as a shared library.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅