Increasing `carState.cumLagMs` on master

### Describe the bug

Re-posting per discussions from Discord:

> Seeing `carState.cumLagMs` doing this linear increase after our recent upstream syncs, also see this in some stock openpilot routes. Is this expected moving forward?
>
> Stock openpilot routes:
> - https://connect.comma.ai/424b2a6c1b69a3bc/0000006c--00a1917b4c/660/720
> - https://connect.comma.ai/424b2a6c1b69a3bc/00000059--5e422f076c/5076/5094
>
> Seems to be coming from https://github.com/commaai/openpilot/pull/38464. Once this commit is reverted, `carState.cumLagMs` is no longer increasing.
>
> Some users are reporting LKAS error from lagging.

### Provide a route where the issue occurs

424b2a6c1b69a3bc/0000006c--00a1917b4c

### openpilot version

master

### Additional info

_No response_

Start by reviewing pull request 38464 and the changes affecting carState.cumLagMs. Replay route 424b2a6c1b69a3bc/0000006c--00a1917b4c and compare the signal with the reported reverted behavior. Done means cumLagMs no longer increases linearly on affected stock routes and LKAS lag errors are not triggered.

Written by the indexing model from the issue text.