[rigtorp]

Summary

• Ports the 5 timing tests (constant-interval, constant-interval-ge1, high-speed, low-speed, normal-speed) from Expect to Python 3 in test/timing-validation.py.
• Removes the external dependency on expect and /usr/bin/time.
• Adds automatic loopback interface detection (lo on Linux, lo0 on macOS).
• Uses time.perf_counter() with a sensible ±0.25s margin to prevent flakiness under virtualization.
• Enables timing tests to run on macOS as well as Linux in CI and locally.
• Updates CMakeLists.txt, GitHub Actions workflow, and README.md.