Safety - rapid torque reduction not blocked

### Panda Safety.

1. **Uncontrolled Rapid Torque Reduction**. Currently, `desired_torque` can drop from `.max_torque` to `0` and TX will not be blocked. Test: https://github.com/commaai/opendbc/pull/2398/files. In such scenario steering wheel will center itself which is not safe. - Ideally, when such violation is detected, the torque should be ramped down, not blocked.

2. **Misleading `.max_rate_down`**. Currently TX will be blocked only [when](https://github.com/commaai/opendbc/blob/be2e291f433549d52cb69cfed2b2c0c8b9635075/opendbc/safety/lateral.h#L18-L19) torque is unwinded with the rate **_below_** (not exceeding) the `.max_rate_down` - (when `torque_meas` or `torque_driver` limits are low).

Related:
https://github.com/commaai/opendbc/pull/3652

Start with the safety check in lateral.h referenced by the issue and inspect the test changes in pull request 2398. Trace how desired_torque, torque_meas, and torque_driver are handled during rapid reductions and rate checks. Done means the unsafe behavior is covered by tests and the intended TX blocking or ramp-down behavior is unambiguous.

Written by the indexing model from the issue text.