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/frames/frame-1.svg

@@ -5,13 +5,13 @@
   <path d="M 136.0 52.0 L 164.0 70.0 L 164.0 70.0 L 136.0 52.0 Z" fill="#c5cad4" stroke="#c5cad4" stroke-width="8" stroke-linecap="round" stroke-linejoin="round"/>
   <path d="M 164.0 70.0 L 164.0 128.0" stroke="#c5cad4" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="leg_l" d="M 135.9 124.8 L 179.4 110.0 L 180.0 149.4 L 191.0 149.2" stroke="#a9afba" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
-  <path id="arm_l" d="M 140.3 40.5 L 164.9 57.4 L 180.8 82.5" stroke="#86cfc5" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_l" d="M 140.3 40.5 L 164.9 57.4 L 186.9 77.6" stroke="#86cfc5" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="girdle" d="M 140.3 40.5 L 134.4 40.4 L 134.4 42.3" stroke="#3a3f4b" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="pelvisBar" d="M 135.9 124.8 L 130.0 124.0 L 130.0 125.2" stroke="#3a3f4b" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="spine" d="M 130.0 124.0 Q 130.8 81.1 134.4 40.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="leg_r" d="M 130.0 125.2 L 173.8 111.5 L 176.0 150.8 L 187.0 150.2" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
-  <path id="arm_r" d="M 134.4 42.3 L 159.0 59.3 L 174.9 84.3" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
-  <path d="M 177.9 89.4 L 177.9 77.4" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="arm_r" d="M 134.4 42.3 L 159.0 59.3 L 180.9 79.4" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <path d="M 183.9 84.5 L 183.9 72.5" stroke="#6b7180" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
   <circle id="head" cx="137.2" cy="20.9" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
   <line id="nose" x1="146.7" y1="24.0" x2="153.4" y2="26.1" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">

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]}