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/Arm Curl/frames/frame-2.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Arm Curl</title>
-  <rect x="16" y="150.8" width="288" height="10.4" rx="3" fill="none" stroke="#b9bec9" stroke-width="3"/>
+  <path d="M 115.2 160.7 L 203.0 160.7 L 203.0 150.3 L 115.2 150.3 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <path d="M 116.0 134.0 L 150.0 134.0" stroke="#c5cad4" stroke-width="8" stroke-linecap="round" stroke-linejoin="round"/>
   <path d="M 136.0 52.0 L 164.0 70.0" stroke="#c5cad4" stroke-width="8" stroke-linecap="round" stroke-linejoin="round"/>
   <path d="M 164.0 70.0 L 164.0 128.0" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>