bench/real-probe.ts steps one joint a little, reports arrive time, final error and overshoot, refuses while a turn is held, and always restores the start pose #517

From #439: the steward's move-probe.ts, moved into the repo. Branch smith/439-precision-forever-calibrate-the-model-to has a version. Take only the probe and its test.

Done when:

• [ ] bun run Software/so101/bench/real-probe.ts <url> --joint N [--step ticks] drives one joint over /follow, samples /status at about 50 Hz, and prints time to arrive within 12 ticks, final error and overshoot.
• [ ] It refuses to start unless /status shows nobody driving, no STOP and no E-stop. If any of these appears mid-run it stops at once.
• [ ] After every run, including an aborted one, it returns the joint to its start pose, so a gravity joint cannot walk toward its limit. The step is small by default and clamped to the joint's limits.
• [ ] bench/real-probe.test.ts runs it against the fake server: numbers come out, the joint ends at its start pose, and a held turn is refused.
• [ ] Software/README.md has one short paragraph with the exact command. forge.json is untouched.

Risk: moving the real arm, which the refusals and the restore cover. Brief: any builder.