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/Barbell Squat/frames/frame-1.svg

@@ -8,9 +8,9 @@
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-  <circle cx="183.8" cy="36.0" r="9.799999999999999" fill="#6b7180"/>
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Exercise Library/Barbell Squat/frames/frame-2.svg

@@ -8,9 +8,9 @@
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Exercise Library/Barbell Squat/motion.json

@@ -4,7 +4,7 @@
   "camera": {"zoom": 0.7},
   "working": ["leg_r", "leg_l"],
   "props": [
-    {"type": "bar", "at": ["hand_r", "hand_l"], "halfLen": 26, "plateR": 14}
+    {"type": "bar", "axis": "z", "at": ["hand_r", "hand_l"], "halfLen": 26, "plateR": 14}
   ],
   "frames": [
     {

Exercise Library/Barbell Squat/visual.svg

@@ -8,9 +8,9 @@
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