feat: expose macOS gs_usb device discovery

[TomCC7]

Summary

• expose filtered gs_usb enumeration from the native macOS transport
• add typed Rust and Python device descriptors with stable serial selectors
• document and smoke-test the Python discovery surface

Validation

• cargo test -p can-motor-control
• cargo test -p can-motor-control-py --no-run
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --check