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/Triceps Pushdown/frames/frame-1.svg

@@ -10,7 +10,7 @@
   <path id="pelvisBar" d="M 157.1 92.3 L 153.0 91.8 L 153.0 92.7" stroke="#3a3f4b" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="spine" d="M 153.0 91.8 Q 156.7 62.0 162.3 33.9" stroke="#3a3f4b" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 162.3 35.3 L 151.5 53.0 L 168.4 40.7" stroke="#0d9488" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 162.8 40.1 L 176.8 40.1" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 169.8 38.9 L 169.8 41.3" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="166.2" cy="20.7" r="7.0" fill="white" stroke="#3a3f4b" stroke-width="4.2"/>
   <line id="nose" x1="173.1" y1="21.6" x2="177.9" y2="22.3" stroke="#3a3f4b" stroke-width="2.8" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">

Exercise Library/Triceps Pushdown/frames/frame-2.svg

@@ -10,7 +10,7 @@
   <path id="pelvisBar" d="M 157.1 92.3 L 153.0 91.8 L 153.0 92.7" stroke="#3a3f4b" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="spine" d="M 153.0 91.8 Q 156.7 62.0 162.3 33.9" stroke="#3a3f4b" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="arm_r" d="M 162.3 35.3 L 158.8 55.7 L 165.6 75.2" stroke="#0d9488" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 160.0 74.6 L 174.0 74.6" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 167.0 73.4 L 167.0 75.8" stroke="#6b7180" stroke-width="2.8" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="166.2" cy="20.7" r="7.0" fill="white" stroke="#3a3f4b" stroke-width="4.2"/>
   <line id="nose" x1="173.1" y1="21.6" x2="177.9" y2="22.3" stroke="#3a3f4b" stroke-width="2.8" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">