bench/limits-sweep.ts drives a fixed per-joint sweep and prints the /precision scorecard before and after, against the fake bus or a given arm URL #478

From #438 step 4. This gives #469 a sweep anyone can rerun.

Done:

• [ ] bun Software/so101/bench/limits-sweep.ts [url] drives each joint in turn through the same fixed targets (a documented list inside the joint limits, with the others held still). It then prints that joint's p95 tracking error, time to arrive, overshoot and snap-backs from /precision, one line per joint.
• [ ] With no url, it runs against the fake bus with the real Governor, as bench/precision.ts does, so the crew can run it anywhere.
• [ ] With a url, it refuses to start unless the lobby shows nobody driving, and it stops at once on STOP or on any refusal.
• [ ] A test runs it on the fake bus and checks there is one line per joint and that the output is the same from run to run.
• [ ] Software/README.md has two lines on how to run it.