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/Leg Extension/frames/frame-1.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Leg Extension</title>
-  <rect x="16" y="150.8" width="288" height="10.4" rx="3" fill="none" stroke="#b9bec9" stroke-width="3"/>
+  <path d="M 75.2 160.6 L 221.1 160.6 L 221.1 150.2 L 75.2 150.2 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <path d="M 110.0 122.0 L 86.0 48.0" stroke="#c5cad4" stroke-width="9" stroke-linecap="round" stroke-linejoin="round"/>
   <path d="M 104.0 124.0 L 166.0 120.0" stroke="#c5cad4" stroke-width="9" stroke-linecap="round" stroke-linejoin="round"/>
   <path d="M 108.0 126.0 L 108.0 150.0" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>