Rivian: remove stalk-down hold requirement for ACC set speed update

[Geomglot]

Description

This fixes Rivian ACC set-speed handling so a stalk-down action updates the set speed immediately instead of requiring an approximately 0.5 second hold
first.

The previous behavior added delay that did not match observed stock Rivian behavior and could briefly leave the comma display and the Rivian cluster
showing different set speeds.

This change removes the stalk_down_counter hold requirement in Rivian carstate_ext.py. A follow-up comment cleanup is included in the same branch.

Verification

Verified by:

• on-road testing in a Rivian confirming stalk-down no longer requires a hold before set speed updates
• comparison against expected Rivian driver behavior

[Geomglot]

Small Rivian bugfix PR.

This change is limited to opendbc/sunnypilot/car/rivian/carstate_ext.py and removes the previous stalk-down hold requirement while keeping the snap-to-
current-speed behavior gated on an active stalk-down request.

Checks are now passing, including safety and the full ./test.sh workflow.

[Geomglot]

Note that this is incorporated into #476 as well because the code is intertwined - but this could stand on its own pending 476 being merged

[sunnyhaibin]

@Geomglot do you have a test route with this tested?

[Geomglot]

I just created this one. https://connect.comma.ai/4440a486580ed7c6/00000003--2f49b04818

Test was right around the end of segment 0 start of segment 1