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/Arm Curl/motion.json

@@ -16,8 +16,8 @@
       "root": {"pos": [130, 124], "pitch": 2},
       "spine": [0, 2],
       "neck": 4, "head": 10,
-      "shoulder_r": 59, "elbow_r": -23,
-      "shoulder_l": 59, "elbow_l": -23,
+      "shoulder_r": 59, "elbow_r": -8,
+      "shoulder_l": 59, "elbow_l": -8,
       "hip_r": 84, "knee_r": 64,
       "hip_l": 84, "knee_l": 64,
       "pins": {"foot_r": [176, 150], "foot_l": [180, 151]}