feat: add discrete RV subtraction and division operators via FastRV

[MthwRobinson]

Description

This PR expands discrete random variable algebra across the Rust core and the Python API.

On the Rust side, it adds Sub, SubAssign, Div, DivAssign, and MulAssign support for
RandomVariable, using the discrete transform machinery to implement subtraction by negating the RHS
and division by multiplying by the transformed reciprocal. It also adds validation for discrete
transformations, including checks for empty transformation lists, invalid or non-adjacent ranges,
incomplete support coverage, and divide-by-zero support in RV division.

To support those operations, shared sorting and support de-duplication logic was extracted into a new
shared module and reused by convolution, product, mixture, and transform code. The branch also adds
doc tests and unit coverage for the new algebra operations and the shared helpers.

On the Python side, RV now routes discrete RV-to-RV +, -, *, and / operations through
FastRV, and the Rust-backed Python API now exposes the corresponding reflected and direct operator
methods. Error handling was updated so divide-by-zero and similar Rust-side failures surface as
ValueError, and the Python lint target was adjusted to run ruff with uv --no-sync.

Testing

Run the Rust checks and tests:

• make cargo-check
• make cargo-test

Run the Python checks and tests:

• make check
• make rust-develop
• make test