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/Calfs/motion.json

@@ -33,8 +33,8 @@
       "neck": 16,
       "shoulder_r": 44, "elbow_r": 25,
       "shoulder_l": 44, "elbow_l": 25,
-      "hip_r": 66, "knee_r": -18, "ankle_r": -18,
-      "hip_l": 66, "knee_l": -18, "ankle_l": -18,
+      "hip_r": 66, "knee_r": -8, "ankle_r": -18,
+      "hip_l": 66, "knee_l": -8, "ankle_l": -18,
       "pins": {"hand_r": [166, 74], "hand_l": [170, 76], "foot_r": [212, 139], "foot_l": [216, 141]}
     }
   ]