feat: add collision checking option to IK solver

[jimsynz]

Summary

Adds optional self-collision detection to reject IK solutions that would cause the robot to collide with itself.

New Options

Option	Type	Default	Description
:check_collisions	boolean	false	Enable collision checking after IK converges
:collision_margin	float	0.0	Safety margin in metres for collision detection

Usage

# Basic collision checking
DLS.solve(robot, positions, :tip, target, check_collisions: true)

# With safety margin (detects near-misses)
DLS.solve(robot, positions, :tip, target,
  check_collisions: true,
  collision_margin: 0.01
)

Error Handling

When a collision is detected, returns:

{:error, %BB.Error.Kinematics.SelfCollision{
  link_a: :forearm,
  link_b: :base,
  joint_positions: %{shoulder: 1.2, elbow: -2.1}
}}

Dependencies

Requires bb ~> 0.11 with collision detection support (beam-bots/bb#25).

Test plan

• Default behaviour unchanged (no collision checking)
• Collision-free solutions accepted when checking enabled
• Self-collision returns SelfCollision error
• Collision margin parameter respected
• New CollisionArm test robot with collision geometry