Robot descriptions v2.19.0

RAFT, the teleoperation leader arm for PAR6

This release adds RAFT, Source Robotics' teleoperation leader arm: PAR6's kinematic structure at 5/9 scale, with a
trigger on its handle that drives the follower's gripper.

• Two new packages: mujoco/raft and urdf/raft_description, downloadable as raft_mujoco.zip and
  raft_urdf.zip. They use PAR6's joint names, so RAFT's joint angles map one to one onto PAR6's. The MuJoCo model
  has torque actuators (RAFT's servos run in current mode), the trigger, the optional J2 counterbalance tendon and
  home / parked keyframes. The URDF package has native ROS 2 display support:
  ros2 launch raft_description display.launch.py.
• Built from RAFT's own model reference (source/raft/), the same source as RAFT's firmware model. Both formats
  are checked against its forward kinematics and gravity torques. One end of every joint range is a mechanical stop
  measured on the first arm; the other comes from a sweep of the CAD.
• Colours from the shared palette: RAFT's printed parts, servos, horns and screws are separate visual pieces.
  New keys raft_structure, raft_trigger, raft_horns and raft_fasteners; RAFT's servos follow motors or the
  new raft_motors override.
• Guides and checks: a RAFT product guide and a RAFT column in "Meet the models"; the Linux ROS 2 checks, the
  packaging and the release include raft_description.

PAR6, PAROL6, MSG and SSG48 are unchanged: geometry, colors, kinematics, inertia, collision shapes, joint limits and
actuation settings stay as in v2.18.1.

Models/data/docs remain CC BY 4.0; support code remains Apache-2.0.