Speed and acceleration limits can differ per joint: maxSpeedDegS and maxAccelDegS2 take a per-joint override, defaults unchanged #477

From #438 step 4. #469 needs this before anyone can raise one joint's limits without raising them all.

Done:

• [ ] Software/so101/src/safety.ts accepts an optional jointSpeedDegS and jointAccelDegS2, maps of joint name to number, next to maxSpeedDegS and maxAccelDegS2. The Governor uses a joint's own value when it has one, and the global value otherwise.
• [ ] PUT /settings validates them at the boundary: known joint names, finite numbers, and no higher than the documented ceiling that applies to the global limits. They are saved and returned like jointLimitsDeg.
• [ ] With nothing set, every motion is exactly what it is today. A test proves it with a fixed trace, and another proves that a joint with its own lower limit is held to it.
• [ ] STOP, the zones and the gripper torque cap are untouched.
• [ ] bun --cwd Software/so101 test passes.