feat: add BB.Sensor.Mimic for mechanically-linked joints

[jimsynz]

Summary

• Adds BB.Sensor.Mimic sensor that derives joint state from another joint
• Subscribes to source joint's sensor messages and re-publishes transformed values
• Useful for modelling parallel jaw grippers and other mechanically-coupled joints

Options

Option	Type	Default	Description
source	atom	required	Name of the source joint to follow
multiplier	float	1.0	Scale factor for position values
offset	float	0.0	Constant offset added after scaling
message_types	list	[JointState]	Message types to forward

Example Usage

joint :right_finger do
  type(:prismatic)
  sensor(:mimic, {BB.Sensor.Mimic,
    source: :left_finger,
    multiplier: 1.0,
    offset: 0.0
  })
end

URDF Equivalent

Implements the equivalent of URDF mimic joints:

<joint name="right_finger_joint" type="prismatic">
  <mimic joint="left_finger_joint" multiplier="1" offset="0"/>
</joint>

Formula: mimic_position = source_position * multiplier + offset

Test plan

• Unit tests for init, message transformation, and forwarding
• Verified with bb_example_wx200 parallel jaw gripper
• All existing tests pass
• mix check passes