added panic_across_ffi example demonstrating panic crossing FFI boundary

[vikashsiwach]

Summary

This PR adds a new interop example: panic_across_ffi.

It demonstrates the undefined behavior that occurs when a Rust panic crosses an FFI boundary into C++, and shows the correct way to handle such cases safely.

What This Example Demonstrates

This example includes two scenarios:

Unsafe case (trigger_panic)

• A Rust function marked extern "C" panics

• The panic attempts to unwind across the FFI boundary

• Rust aborts the program with:

  panic in a function that cannot unwind
  

• This represents undefined behavior and must be avoided

Safe case (trigger_panic_safe)

• Uses std::panic::catch_unwind
• Contains the panic within Rust
• Prevents it from crossing into C++
• Program continues execution safely

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Why This Matters

While FFI safety often focuses on layout compatibility, error handling across language boundaries is equally critical.

Rust and C++ have different exception/unwinding models:

• Rust uses panic unwinding
• C++ uses exceptions (or no exceptions)

Crossing these boundaries incorrectly can lead to:

• Undefined behavior
• Program termination
• Hard-to-debug runtime issues

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How to Run

cd examples/panic_across_ffi
./run.sh

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Expected Behavior

• The safe function runs successfully and handles the panic internally
• The unsafe function causes the program to abort

This behavior is intentional and demonstrates the problem clearly.

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Notes

• The unsafe example is included for educational purposes
• The abort behavior is expected and correct
• The example follows the repository pattern of demonstrating both incorrect and correct approaches

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CI Compliance

• Uses run.sh for building and running the example
• Passes cargo build, cargo test, cargo clippy, cargo doc, and cargo fmt
• Does not introduce any tracked binaries or build artifacts

---

Related Problem Area

• FFI safety
• Cross-language error handling
• Panic/unwinding behavior across boundaries

[vikashsiwach]

The CI failure in the "Run Rust and C++ examples" step was due to the intentional abort caused by the unsafe example (trigger_panic), where a Rust panic occurs inside an extern "C" function.
Since Rust does not allow unwinding across FFI boundaries, it aborts the program with:
panic in a function that cannot unwind.
This behavior is expected and is part of what the example is demonstrating.
To ensure CI passes while still preserving this behavior, I updated run.sh to allow the program to abort without failing the script:
./example || true

[teor2345]

Hi, I've already looked at your PR #24, we're just going to focus on one PR per person for now. After that PR is finished, we can decide what to do with this one.

[teor2345]

This is a good example, it just needs tidying up

[teor2345]

Thanks, this is a good example!
I've made some minor cleanups on your branch, and improved test coverage.

Sorry it's taken me a while to get back to this, I didn't get any notifications when it was updated.