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

CHANGELOG.md

@@ -1,5 +1,7 @@
 **July 2026**
 
+The Abdominal figure's arms no longer bend backward at the elbow, and a few other form guides lose their subtly hyperextended joints.
+
 The exercise library more than doubles — deadlifts, squats, lunges, bench presses, dumbbell work, pull-ups, and the cable stations — each with its own animated form guide.
 
 Machine form guides now slowly circle around the figure too — seats, cables, and pads turn with the body, showing every exercise from all sides.