Distance-Based Velocity Scaling for SafetyPositionController

[Joschi3]

Summary

Replaces the binary block_if_too_far toggle with mandatory distance-based velocity scaling when self-collision checks are enabled.

Changes:

• CollisionChecker now returns a CollisionResult struct with both collision flag and minimum clearance distance (previously only a boolean)
• New collision_safety_zone parameter: Between collision_padding and collision_safety_zone, joint velocities are linearly scaled from 0% to 100%. Beyond the safety zone, full speed is allowed. At or below padding, motion is blocked entirely.
• Velocity limiting is always enforced when check_self_collisions is enabled (removed the user-disableable block_if_too_far parameter)