feat(algebra): implement discrete RV product in Rust and wire Python RV multiplication

[MthwRobinson]

Description

This PR moves discrete random-variable product logic from Python to Rust and routes RV multiplication through the Rust-backed FastRV path.

Key changes included in this branch:

• Added a new Rust algebra module with product_discrete for independent discrete RVs, including:
  • conversion of inputs to PDF form,
  • cartesian-product support/function computation,
  • sorting + de-duplication of repeated support values by summing probabilities.
• Added Rust-side operator support:
  • impl Mul for RandomVariable,
  • impl Mul for FastRV,
  • Python-exposed product_discrete binding.
• Registered the new binding in rust/src/lib.rs.
• Updated Python integration:
  • removed legacy _product_discrete NumPy implementation from applpy/algebra.py,
  • made product() use to_fast_rv()/from_fast_rv() for discrete RV multiplication,
  • added RV.to_fast_rv() and RV.from_fast_rv() helpers in applpy/rv.py,
  • reused RV.from_fast_rv() in bootstrap_rv.
• Added/updated tests:
  • Rust tests for discrete product behavior (duplicate support merge, CDF input conversion, output metadata),
  • Python tests for FastRV round-trip conversion and discrete multiplication/error paths.
• Included minor cleanup in Rust error strings (conversion typo fixes).

Testing

Run the following from the repository root:

For Rust changes:

• make cargo-check
• make cargo-test

For Python changes:

• make check
• make rust-develop
• make test