harmonic test
An animated double pendulum traces two connected line segments from a fixed center point, with a white ball at the endpoint whose radius oscillates with time. The tip alternates between white and black fills each time step, creating a strobe effect. Using D3 v6 selections, `range`, and `join`, the code updates SVG `line` and `circle` elements every 2 seconds, incrementing a time variable to drive cosine and sine positions for both arm segments.