Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Build the elbow measurement. Write fit(points, centers) (6 k-means iterations, empty-safe update) and inertia(points, centers) (sum over points of the squared distance to the NEAREST center). Fit with the given starts for k=1, 2, 3, print f"k={k} inertia={...}" (inertia rounded to 2), then print whether the elbow is at k=2 — i.e. whether the k=1→2 drop is more than 10x the k=2→3 drop.