Fix G1 TriHand USD up-axis metadata

The G1 TriHand geometry is authored Z-up, but the stage metadata declared Y-up. Direct Newton import therefore rotated the robot 90 degrees onto its side. Change only the stage upAxis metadata from Y to Z so world -Z gravity remains head-to-feet.

robot_assets/g1_29dof_tri_hand_closed.usd

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:83bf2000ee59f5fd3e6c24cff1d76a4631eedae966e12831f8306cbe5f19fb7a
-size 28161424
+oid sha256:9e61f39e3ace1cd60b3cf9eb7dc5422a02584ad5b23b049d2d792bbbec2b9ddc
+size 28163282