Fix figure IK snapping and gate the library on a fail-hard motion checker

Three solver defects made limbs teleport, twist, or windmill: write-back
angles wrapped at ±180 and lerped the long way around; branch flips landed
on configurations the anatomical write-back cannot represent, silently
pulling pinned extremities off their pins; and the degenerate straight-limb
bend plane fell back to the camera axis instead of the anatomical anterior.
solve_limb now verifies each branch reproduces the solved end before
accepting it, resolve unwraps written-back angles toward the pose they
replace, and the degenerate plane comes from the parent's anterior axis.

render.py --check replays every exercise's full tween loop and fails hard
on six invariants (pin fidelity, continuity, wraps, authored-vs-resolved
drift, ground penetration, resolved ROM); --export refuses to ship a
failing exercise. All 66 motions re-authored or retouched to pass: honest
authored angles where pins used to override them silently, grounded feet
on the seated machines, a vertical bench-press bar path, straight-armed
child's pose, a butterfly stretch seated on the mat, and FK arms where
pins forced impossible reaches. MotionSolver.swift mirrors the solver
changes line for line, held by regenerated fixtures.

Claude-Session: https://claude.ai/code/session_01PKptrgbx74peTwHGRxBojv

Exercise Library/Face Pull/frames/frame-2.svg

@@ -1,22 +1,22 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Face Pull</title>
   <defs>
-    <linearGradient id="grad-leg_l-0" gradientUnits="userSpaceOnUse" x1="157.1" y1="92.3" x2="163.7" y2="123.4"><stop offset="0" stop-color="#525763"/><stop offset="1" stop-color="#a9afba"/></linearGradient>
-    <linearGradient id="grad-leg_r-1" gradientUnits="userSpaceOnUse" x1="160.4" y1="123.5" x2="150.2" y2="149.2"><stop offset="0" stop-color="#3a3f4b"/><stop offset="1" stop-color="#585d69"/></linearGradient>
-    <linearGradient id="grad-leg_r-2" gradientUnits="userSpaceOnUse" x1="150.2" y1="149.2" x2="157.4" y2="151.9"><stop offset="0" stop-color="#585d69"/><stop offset="1" stop-color="#616772"/></linearGradient>
+    <linearGradient id="grad-leg_l-0" gradientUnits="userSpaceOnUse" x1="157.1" y1="92.3" x2="165.7" y2="122.9"><stop offset="0" stop-color="#535864"/><stop offset="1" stop-color="#a9afba"/></linearGradient>
+    <linearGradient id="grad-leg_r-1" gradientUnits="userSpaceOnUse" x1="160.4" y1="123.5" x2="150.2" y2="149.2"><stop offset="0" stop-color="#3a3f4b"/><stop offset="1" stop-color="#595e6a"/></linearGradient>
+    <linearGradient id="grad-leg_r-2" gradientUnits="userSpaceOnUse" x1="150.2" y1="149.2" x2="157.4" y2="151.9"><stop offset="0" stop-color="#595e6a"/><stop offset="1" stop-color="#626873"/></linearGradient>
   </defs>
   <path d="M 141.8 157.5 L 209.0 157.5 L 209.0 150.2 L 141.8 150.2 Z" fill="none" stroke="#b9bec9" stroke-width="2.1" stroke-linejoin="round"/>
   <circle cx="217.4" cy="24.6" r="2.8" fill="#6b7180"/>
   <path d="M 217.4 24.6 L 168.6 29.2" stroke="#c5cad4" stroke-width="2.0999999999999996" stroke-linecap="round" stroke-linejoin="round"/>
   <g id="leg_l">
-    <path d="M 157.1 92.3 L 163.7 123.4" stroke="url(#grad-leg_l-0)" stroke-width="3.77463" stroke-linecap="round"/>
-    <path d="M 163.7 123.4 L 153.0 148.9" stroke="#a9afba" stroke-width="3.5" stroke-linecap="round"/>
-    <path d="M 153.0 148.9 L 160.1 151.8" stroke="#a9afba" stroke-width="3.5" stroke-linecap="round"/>
+    <path d="M 157.1 92.3 L 165.7 122.9" stroke="url(#grad-leg_l-0)" stroke-width="3.77104" stroke-linecap="round"/>
+    <path d="M 165.7 122.9 L 153.0 147.5" stroke="#a9afba" stroke-width="3.5" stroke-linecap="round"/>
+    <path d="M 153.0 147.5 L 159.9 150.9" stroke="#a9afba" stroke-width="3.5" stroke-linecap="round"/>
   </g>
   <g id="leg_r">
     <path d="M 153.0 92.7 L 160.4 123.5" stroke="#3a3f4b" stroke-width="4.2" stroke-linecap="round"/>
-    <path d="M 160.4 123.5 L 150.2 149.2" stroke="url(#grad-leg_r-1)" stroke-width="4.10551" stroke-linecap="round"/>
-    <path d="M 150.2 149.2 L 157.4 151.9" stroke="url(#grad-leg_r-2)" stroke-width="3.98151" stroke-linecap="round"/>
+    <path d="M 160.4 123.5 L 150.2 149.2" stroke="url(#grad-leg_r-1)" stroke-width="4.10192" stroke-linecap="round"/>
+    <path d="M 150.2 149.2 L 157.4 151.9" stroke="url(#grad-leg_r-2)" stroke-width="3.97434" stroke-linecap="round"/>
   </g>
   <g id="arm_l">
     <path d="M 163.3 33.6 L 178.6 45.9" stroke="#86cfc5" stroke-width="3.5" stroke-linecap="round"/>