Add jeff program verification helpers

[Forage409]

Closes #57.

This adds a small verification module to the Rust reader API:

• verify(&impl ReadJeff) and verify_module(Module) entry points
• schema major-version and entrypoint validation
• function signature value resolution checks
• dataflow checks for values used before they are defined
• nested structured-control-flow region checks
• function-call target validation
• tests covering the existing example programs plus an invalid entrypoint case

Tested with:

CARGO_HTTP_CHECK_REVOKE=false cargo test -q

Disclosure: I used AI assistance while implementing and checking this change.

[denialhaag]

Thanks for your interest in contributing to jeff, @Forage409! 🙂

Before I can review this, please add more tests (at least one negative test per bullet point in #57, but more is always better). That way, we both get a sense of what your implementation already covers. If you need help with generating jeff programs, see encode_examples.sh or generate_test_cases.py.

[denialhaag]

We have decided to go with #68 to resolve #57, so I'll close this PR.

Thanks again for your interest in contributing to jeff, @Forage409! 🙂