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

[MthwRobinson]

Description

This PR adds a Rust implementation of discrete truncation and integrates it into the Python transform flow.

Between main and this feature branch, the change set introduces a new transform::truncate_discrete algorithm in Rust that:

• converts inputs to PDF form,
• validates truncation bounds,
• computes retained mass in the requested support range,
• renormalizes probabilities,
• returns clear errors for invalid bounds or zero retained mass.

The Rust function is exported through the Python bindings (truncate_discrete), registered in the extension module, and then used by applpy/transform.py to replace the previous pure-Python discrete truncation path.

The PR also:

• adds a strict import-time error message in Python when applpy_rust is not built,
• includes Rust doc-test coverage and Rust unit tests for truncation behavior and bound validation,
• updates Python unit expectations so invalid equal bounds now raise ValueError.

Testing

Run the following from the repository root:

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