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/motion.json

@@ -19,7 +19,7 @@
       "root": {"pos": [174, 137], "yaw": 180, "pitch": 80},
       "spine": [0, 0],
       "neck": 0, "head": -80,
-      "shoulder_r": -90, "elbow_r": 0,
+      "shoulder_r": -70, "elbow_r": 0,
       "shoulder_l": 80, "elbow_l": 90,
       "hip_r": 0, "knee_r": 0,
       "hip_l": 0, "knee_l": 0,