Add pi05_droid_jointpos config serving absolute joint actions

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Summary

• Add the pi05_droid_jointpos train config: DROID with absolute joint-position actions — the recipe RoboLab's leaderboard serves for its pi0-family policies (their policies/pi0_family/README.md runs the xuningy/openpi fork; this ports the one difference into ours).
• It is pi05_droid plus an AbsoluteActions(make_bool_mask(7, -1)) output transform: the model still predicts joint deltas, the transform adds back the observed joint position, so served chunks are absolute joint positions (gripper stays absolute).
• Checkpoint: gs://openpi-assets-simeval/pi05_droid_jointpos (action_horizon=15).

Test plan

• Config instantiates and serves the pinned checkpoint via scripts/serve_policy.py.
• Used as the serving config for the RoboLab differential eval in the companion positronic PR (three tasks, 10 episodes per arm; positronic wire vs RoboLab's own pi0_family client against this same server process) — success rates matched per task.