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/Shoulder Press/frames/frame-1.svg

@@ -12,7 +12,7 @@
   <path id="spine" d="M 120.0 130.0 Q 103.0 90.4 95.1 50.1" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="leg_r" d="M 120.0 131.2 L 161.9 112.6 L 164.0 151.9 L 175.0 151.3" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 95.1 52.0 L 110.1 77.6 L 110.1 48.0" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 103.1 47.1 L 123.1 47.1" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 113.1 45.4 L 113.1 48.8" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="98.6" cy="30.7" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
   <line id="nose" x1="108.5" y1="29.3" x2="115.4" y2="28.3" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">

Exercise Library/Shoulder Press/frames/frame-2.svg

@@ -12,7 +12,7 @@
   <path id="spine" d="M 120.0 130.0 Q 103.0 90.4 95.1 50.1" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="leg_r" d="M 120.0 131.2 L 161.9 112.6 L 164.0 151.9 L 175.0 151.3" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 95.1 52.0 L 112.8 28.1 L 133.6 6.8" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 126.6 5.9 L 146.6 5.9" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 136.6 4.2 L 136.6 7.6" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="98.6" cy="30.7" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
   <line id="nose" x1="108.5" y1="29.3" x2="115.4" y2="28.3" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">

Exercise Library/Shoulder Press/motion.json

@@ -9,7 +9,7 @@
       {"kind": "line", "pts": [[126, 143], [126, 151]], "w": 5},
       {"kind": "line", "pts": [[110, 151], [142, 151]], "w": 4, "depth": 5}
     ]},
-    {"type": "bar", "at": ["hand_r", "hand_l"], "halfLen": 10}
+    {"type": "bar", "axis": "z", "at": ["hand_r", "hand_l"], "halfLen": 10}
   ],
   "frames": [
     {

Exercise Library/Shoulder Press/visual.svg

@@ -12,7 +12,7 @@
   <path id="spine" d="M 120.0 130.0 Q 103.0 90.4 95.1 50.1" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="leg_r" d="M 120.0 131.2 L 161.9 112.6 L 164.0 151.9 L 175.0 151.3" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 95.1 52.0 L 112.8 28.1 L 133.6 6.8" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 126.6 5.9 L 146.6 5.9" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 136.6 4.2 L 136.6 7.6" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="98.6" cy="30.7" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
   <line id="nose" x1="108.5" y1="29.3" x2="115.4" y2="28.3" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">