Draw the ground as an exercise mat that rotates with the figure

The floor rectangle was screen-locked, which broke the illusion the
moment the camera orbited. It is now a world-space quad on the ground
plane, sized to each motion's projected footprint across its key frames
and rotated through the same camera as the figure - a long rectangle in
profile, a parallelogram mid-orbit, end-on when face-on. Both renderers
in lockstep; fixtures unaffected (the mat is a pure addition).

Claude-Session: https://claude.ai/code/session_01LEoff8bXGBS83tK1c55Mf7

Exercise Library/Dead Bug/frames/frame-4.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Dead Bug</title>
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+  <path d="M 45.6 161.1 L 285.6 161.1 L 285.6 150.7 L 45.6 150.7 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <path id="arm_l" d="M 105.9 148.1 L 84.7 127.2 L 63.5 106.3" stroke="#86cfc5" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
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   <path id="girdle" d="M 105.9 148.1 L 100.0 148.0 L 100.0 149.9" stroke="#3a3f4b" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>