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/Abductor/frames/frame-1.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Abductor</title>
-  <rect x="16" y="150.8" width="288" height="10.4" rx="3" fill="none" stroke="#b9bec9" stroke-width="3"/>
+  <path d="M 130.0 145.6 L 130.0 165.2 L 190.0 165.2 L 190.0 145.6 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <rect x="152" y="56" width="16" height="72" rx="5" fill="#c5cad4"/>
   <rect x="130" y="126" width="60" height="10" rx="3" fill="#c5cad4"/>
   <path d="M 160.0 136.0 L 160.0 150.0" stroke="#c5cad4" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>

Exercise Library/Abductor/frames/frame-2.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Abductor</title>
-  <rect x="16" y="150.8" width="288" height="10.4" rx="3" fill="none" stroke="#b9bec9" stroke-width="3"/>
+  <path d="M 130.0 145.6 L 130.0 165.2 L 190.0 165.2 L 190.0 145.6 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <rect x="152" y="56" width="16" height="72" rx="5" fill="#c5cad4"/>
   <rect x="130" y="126" width="60" height="10" rx="3" fill="#c5cad4"/>
   <path d="M 160.0 136.0 L 160.0 150.0" stroke="#c5cad4" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>

Exercise Library/Abductor/visual.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Abductor</title>
-  <rect x="16" y="150.8" width="288" height="10.4" rx="3" fill="none" stroke="#b9bec9" stroke-width="3"/>
+  <path d="M 130.0 145.6 L 130.0 165.2 L 190.0 165.2 L 190.0 145.6 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <rect x="152" y="56" width="16" height="72" rx="5" fill="#c5cad4"/>
   <rect x="130" y="126" width="60" height="10" rx="3" fill="#c5cad4"/>
   <path d="M 160.0 136.0 L 160.0 150.0" stroke="#c5cad4" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>