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/Goblet Squat/motion.json

@@ -12,24 +12,24 @@
       "root": {"pos": [160, 66], "pitch": 5},
       "spine": [0, 0],
       "neck": 2, "head": -6,
-      "shoulder_r": 38, "elbow_r": 125,
-      "shoulder_l": 38, "elbow_l": 125,
+      "shoulder_r": 5, "elbow_r": 140,
+      "shoulder_l": 5, "elbow_l": 140,
       "hip_r": 5, "knee_r": 4, "ankle_r": 0,
       "hip_l": 5, "knee_l": 4, "ankle_l": 0,
       "pins": {"foot_r": [162, 148], "foot_l": [166, 150],
-               "hand_r": [176, -2], "hand_l": [180, 0]}
+               "hand_r": [185, -8], "hand_l": [189, -6]}
     },
     {
       "hold": 0.4, "tween": 1.0,
       "root": {"pos": [150, 104], "pitch": 22},
       "spine": [0, -4],
       "neck": 4, "head": -18,
-      "shoulder_r": 46, "elbow_r": 120,
-      "shoulder_l": 46, "elbow_l": 120,
+      "shoulder_r": 15, "elbow_r": 135,
+      "shoulder_l": 15, "elbow_l": 135,
       "hip_r": 96, "knee_r": 116, "ankle_r": 20,
       "hip_l": 96, "knee_l": 116, "ankle_l": 20,
       "pins": {"foot_r": [162, 148], "foot_l": [166, 150],
-               "hand_r": [190, 40], "hand_l": [194, 42]}
+               "hand_r": [196, 36], "hand_l": [200, 38]}
     }
   ]
 }