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

Workouts/Resources/ExerciseMotions/Abdominal.motion.json

@@ -18,8 +18,8 @@
       "root": {"pos": [128, 126], "pitch": -4},
       "spine": [0, 4],
       "neck": 8,
-      "shoulder_r": 78, "elbow_r": -40,
-      "shoulder_l": 78, "elbow_l": -40,
+      "shoulder_r": 78, "elbow_r": 40,
+      "shoulder_l": 78, "elbow_l": 40,
       "hip_r": 66, "knee_r": 70,
       "hip_l": 66, "knee_l": 70,
       "pins": {"hand_r": [155, 62], "hand_l": [161, 64], "foot_r": [168, 150], "foot_l": [172, 150]}
@@ -29,8 +29,8 @@
       "root": {"pos": [128, 126], "pitch": 10},
       "spine": [0, 40],
       "neck": 18, "head": -26,
-      "shoulder_r": 128, "elbow_r": -40,
-      "shoulder_l": 128, "elbow_l": -40,
+      "shoulder_r": 128, "elbow_r": 40,
+      "shoulder_l": 128, "elbow_l": 40,
       "hip_r": 80, "knee_r": 70,
       "hip_l": 80, "knee_l": 70,
       "pins": {"hand_r": [190, 76], "hand_l": [196, 78], "foot_r": [168, 150], "foot_l": [172, 150]}