Add support for qpos/pose/rot6d conversions

[k1000dai]

Fix #32

Summary

OpenArm Dataset stores only the raw recorded arm state (qpos from joint
teleop, pose from Cartesian teleop). This PR adds on-the-fly conversion
between state representations, so any dataset can be consumed — or exported
to LeRobot — in the representation a policy needs, without ever rewriting
the raw data.

• qpos → pose via forward kinematics, pose → qpos via inverse
  kinematics, using openarm_control
  (MuJoCo/mink) as a new core dependency
• rot6d (position + first two rotation-matrix columns + gripper, 10-dim,
  Zhou et al.'s continuous 6D rotation representation) derived from the pose
• LeRobot v2.1 / v3.0 / GR00T exports always emit one uniform representation,
  qpos by default

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[k1000dai]

enactic/openarm_control#18
pose / qpos conversion needs to match this convention. pose origin is arm_origin in mjcf.