Draw both arms on the supine motions and retire the hide list

Hollow Body Hold, Leg Raises, and Reverse Crunch hid the far arm
because it was never authored - a leftover from the planar rig. Both
arms are now posed, so the far arm reads as the standard light member
behind the near one, the same visual language every other exercise
uses, and the figures stay truthful from any viewpoint. With no
remaining users, the hide mechanism is deleted from both renderers,
the motion schema, and the docs.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/Reverse Crunch/frames/frame-1.svg

@@ -6,6 +6,7 @@
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   <path id="pelvisBar" d="M 177.9 148.8 L 172.0 148.0 L 172.0 149.2" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="spine" d="M 172.0 148.0 Q 128.5 148.0 87.0 148.0" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_r" d="M 87.0 149.9 L 117.0 150.9 L 147.0 152.0" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="leg_r" d="M 172.0 149.2 L 164.0 104.6 L 201.6 118.1 L 205.4 107.9" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="67.4" cy="143.9" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
   <line id="nose" x1="67.4" y1="133.9" x2="67.4" y2="127.0" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>