Publish prebuilt PyPI wheels and fix Ctrl+C cancellation

[jamesdborin]

Summary

• publish prebuilt Rust-extension wheels for Linux/macOS/Windows in the Python release workflow
• switch PyPI publish step to upload wheel artifacts from matrix jobs (no sdist fallback builds)
• make benchmark runs cancel on Ctrl+C by listening for SIGINT and aborting worker tasks
• release the Python GIL during long-running Rust calls in PyO3 bindings so installed batchbench reacts to interrupts promptly

Validation

• cargo check --manifest-path rust/Cargo.toml
• cargo test --manifest-path rust/Cargo.toml
• cargo check --manifest-path rust/Cargo.toml --features python
• built wheel with maturin and installed locally:
  • maturin build --release --manifest-path rust/Cargo.toml --features python --out dist
  • Python API smoke test (batchbench.run_benchmark(...) in dry-run mode)
  • CLI smoke test (batchbench ... --dry-run)
• manual SIGINT verification for both Rust binary and Python entrypoint (python -m batchbench): process exits quickly after Ctrl+C with partial summary