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

@@ -12,7 +12,7 @@
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   <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"/>
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   <circle id="head" cx="98.6" cy="30.7" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
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