feat(transform): add Rust discrete-mixture transform and wire it into Python API

[MthwRobinson]

Description

This PR adds a Rust implementation of discrete mixture transforms and switches Python’s _mixture_discrete path to use the Rust binding for faster and more consistent behavior.

What changed

• Added mixture_discrete in rust/src/algorithms/transform.rs:
  • validates input lengths (random_variables vs mix_weights)
  • validates mixture weights sum to 1 (with tolerance)
  • combines duplicate support points across component RVs
  • sorts output support and returns a discrete PDF RV
• Added Sum support for Number in rust/src/algorithms/number.rs to support cleaner weight-sum validation.
• Exposed the new transform through PyO3:
  • added mixture_discrete_py in rust/src/python/api.rs
  • registered it in rust/src/lib.rs
• Updated applpy/transform.py so _mixture_discrete calls applpy_rust.mixture_discrete(...) and returns an RV from the Rust result.
• Added Rust coverage:
  • doc test example for mixture_discrete
  • unit tests for happy path, length mismatch, and invalid weight sums

Overall, the branch replaces Python-side discrete mixture accumulation with a Rust-backed implementation and adds validation/tests around the new behavior.

Testing

Run the following from the repository root:

• make cargo-check
• make cargo-test
• make check
• make rust-develop
• make test