Give cross-body bars a true 3D axis and fix the goblet squat hold

The orbit was always a real camera orbit — figure and props share one
rigid rotation — but a bar's screen-space angle authored the wrong 3D
rod: the default horizontal encoded a rod along the body axis, so
barbells hovered fixed on screen and vanished at the head-on view
where they should span widest. Line props now take "axis": "z" (both
renderers in lockstep, fixture-pinned): the world left-right direction
projects through the camera pitch like the floor quad — end-on plates
in profile, full span face-on, swinging with the hands in between.
Applied to the ten cross-body bars; vertical handles were already
orbit-invariant.

Goblet Squat's hand pins sat so close to the shoulders that the
two-bone IK was degenerate, flipping between a chicken-wing and an
elbow-behind solve; re-pinned level with the shoulders so the elbows
tuck straight down through the whole rep.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/Hip Thrust/motion.json

@@ -7,7 +7,7 @@
       {"kind": "line", "pts": [[50, 122], [88, 122]], "w": 9, "depth": 8},
       {"kind": "line", "pts": [[68, 127], [68, 150]], "w": 5}
     ]},
-    {"type": "bar", "at": ["hand_r", "hand_l"], "halfLen": 26, "plateR": 13}
+    {"type": "bar", "axis": "z", "at": ["hand_r", "hand_l"], "halfLen": 26, "plateR": 13}
   ],
   "frames": [
     {