positonroam: Braking via a fly to vector mode

Activate braking via the fly to vector pathfollower mode with diminishing velocity at the target endpoint.
positionroam: Adding braking timeout

1. manualcontrol: don't clobber the braking state if already in braking mode.
2.pathplanner: We reuse this pathplanner task for pathfollower monitoring of the braking timer. Count down from 50 to 0 at 100ms per tick, means we can brake for up to 5 seconds before transitioning to true positionhold.