feat(kernels): scaffold TVM FFI bidirectional interop test

[peter941221]

1. Summary

This adds the first validation slice for #191 inside pegainfer-kernels.

The patch keeps TVM FFI behind a default-off tvm-ffi-interop feature, adds a small C++ fixture, and adds one ignored integration test that proves both call directions before any production FFI path changes.

2. Invariants

• The default build still does not pull in TVM FFI.
• Existing handwritten CUDA / cuBLAS / FlashInfer / Triton AOT boundaries stay unchanged.
• This PR does not change runtime behavior, scheduler behavior, or model code.
• The first milestone stays CPU-only on the fixture side.

3. Changes

• Add optional tvm-ffi = "0.1.0-alpha.0" to pegainfer-kernels and gate it behind tvm-ffi-interop.
• Add tests/fixtures/tvm_ffi_fixture.cc with four exported functions that cover:
  • direct Rust -> TVM FFI scalar call
  • Rust callback passed into TVM FFI
  • TVM FFI callback into a Rust global function
  • callback error propagation back into Rust
• Add tests/tvm_ffi_bidirectional.rs as an ignored integration test that builds the fixture at test runtime and verifies both call directions plus missing-symbol diagnostics.

4. Validation

cargo fmt --all --check

cargo check --release -p pegainfer-kernels --features tvm-ffi-interop --tests -j 1

cargo test --release -p pegainfer-kernels --features tvm-ffi-interop tvm_ffi_bidirectional -- --ignored --nocapture

5. Related

Related to #191.

[gemini-code-assist]

Code Review

This pull request introduces TVM FFI bidirectional interoperability support to the pegainfer-kernels crate, adding the tvm-ffi dependency, a new tvm-ffi-interop feature, a C++ test fixture, and a comprehensive integration test. Feedback suggests removing the unnecessary cargo:rerun-if-changed directive for the test fixture in build.rs to prevent slow, redundant library rebuilds. Additionally, it is recommended to replace the login shell invocation (sh -lc) with a standard shell (sh -c) in the integration test to improve reliability and performance in CI/CD environments.

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