Fix backward-bending joints in the five legacy motions

Abdominal's pinned hands used elbow -40 as the IK plane hint, drawing
the arms hyperextended (user-reported). Flipping the hint bends the
elbows the natural way while the hands stay on the handles. Same class
of fix for the milder cases: Arm Curl and Shoulder Press elbows and
Calfs knees clamp to -8, Side Plank's raised arm to the -70 ROM cap.
The whole library now passes render.py --strict with zero warnings,
making it a valid verification gate. Fixtures regenerated; 48 tests
green.

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

Exercise Library/Side Plank/visual.svg

@@ -2,7 +2,7 @@
   <title>Side Plank</title>
   <path d="M 48.0 160.9 L 277.9 160.9 L 277.9 150.5 L 48.0 150.5 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <path id="leg_r" d="M 179.9 137.8 L 225.2 145.6 L 264.6 152.5 L 262.7 163.1" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
-  <path id="arm_r" d="M 96.2 122.5 L 101.4 93.4 L 106.6 64.3" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_r" d="M 96.2 122.5 L 111.2 96.9 L 126.2 71.4" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="girdle" d="M 90.3 124.4 L 90.3 122.5 L 96.2 122.5" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="pelvisBar" d="M 174.0 138.2 L 174.0 137.0 L 179.9 137.8" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="spine" d="M 174.0 137.0 Q 131.2 129.6 90.3 122.5" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>