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/Pull-Up/frames/frame-1.svg

@@ -8,7 +8,7 @@
   <path id="spine" d="M 157.2 114.9 Q 157.2 84.9 157.2 56.3" stroke="#3a3f4b" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_l" d="M 161.3 56.3 L 160.4 35.7 L 158.6 15.1" stroke="#86cfc5" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 157.2 57.6 L 156.1 37.0 L 155.8 16.3" stroke="#0d9488" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 140.4 15.7 L 174.0 15.7" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 157.2 12.8 L 157.2 18.6" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="157.7" cy="42.5" r="7.0" fill="white" stroke="#3a3f4b" stroke-width="4.2"/>
   <line id="nose" x1="164.7" y1="42.0" x2="169.6" y2="41.7" stroke="#3a3f4b" stroke-width="2.8" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">

Exercise Library/Pull-Up/frames/frame-2.svg

@@ -8,7 +8,7 @@
   <path id="pelvisBar" d="M 161.3 93.0 L 157.2 92.5 L 157.2 93.4" stroke="#3a3f4b" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="spine" d="M 157.2 92.5 Q 157.2 62.5 157.2 33.9" stroke="#3a3f4b" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 157.2 35.2 L 175.0 24.3 L 155.8 15.9" stroke="#0d9488" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 140.4 15.3 L 174.0 15.3" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 157.2 12.4 L 157.2 18.2" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="158.2" cy="20.2" r="7.0" fill="white" stroke="#3a3f4b" stroke-width="4.2"/>
   <line id="nose" x1="165.1" y1="19.4" x2="170.0" y2="18.9" stroke="#3a3f4b" stroke-width="2.8" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">

Exercise Library/Pull-Up/motion.json

@@ -4,7 +4,7 @@
   "camera": {"zoom": 0.7},
   "working": ["arm_r", "arm_l"],
   "props": [
-    {"type": "bar", "at": ["hand_r", "hand_l"], "halfLen": 24}
+    {"type": "bar", "axis": "z", "at": ["hand_r", "hand_l"], "halfLen": 24}
   ],
   "frames": [
     {

Exercise Library/Pull-Up/visual.svg

@@ -8,7 +8,7 @@
   <path id="pelvisBar" d="M 161.3 93.0 L 157.2 92.5 L 157.2 93.4" stroke="#3a3f4b" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="spine" d="M 157.2 92.5 Q 157.2 62.5 157.2 33.9" stroke="#3a3f4b" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 157.2 35.2 L 175.0 24.3 L 155.8 15.9" stroke="#0d9488" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 140.4 15.3 L 174.0 15.3" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 157.2 12.4 L 157.2 18.2" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="158.2" cy="20.2" r="7.0" fill="white" stroke="#3a3f4b" stroke-width="4.2"/>
   <line id="nose" x1="165.1" y1="19.4" x2="170.0" y2="18.9" stroke="#3a3f4b" stroke-width="2.8" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">