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/Cobra Stretch/visual.svg

@@ -1,33 +1,29 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 180" fill="none">
   <title>Cobra Stretch</title>
-  <defs>
-    <linearGradient id="grad-arm_r-0" gradientUnits="userSpaceOnUse" x1="132.3" y1="90.6" x2="118.7" y2="117.0"><stop offset="0" stop-color="#a5abb6"/><stop offset="1" stop-color="#a9afba"/></linearGradient>
-    <linearGradient id="grad-arm_l-1" gradientUnits="userSpaceOnUse" x1="117.2" y1="120.6" x2="107.1" y2="148.4"><stop offset="0" stop-color="#3a3f4b"/><stop offset="1" stop-color="#404551"/></linearGradient>
-  </defs>
-  <path d="M 62.7 161.1 L 294.1 161.1 L 294.1 150.7 L 62.7 150.7 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
-  <g id="leg_r">
-    <path d="M 193.9 147.8 L 235.1 168.0" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
-    <path d="M 235.1 168.0 L 270.0 148.8" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
-    <path d="M 270.0 148.8 L 280.8 150.8" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
-  </g>
+  <path d="M 41.9 161.1 L 302.0 161.1 L 302.0 150.7 L 41.9 150.7 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <g id="arm_r">
-    <path d="M 132.3 90.6 L 118.7 117.0" stroke="url(#grad-arm_r-0)" stroke-width="5.01723" stroke-linecap="round"/>
-    <path d="M 118.7 117.0 L 104.2 142.9" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 112.6 122.6 L 88.3 139.9" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 88.3 139.9 L 59.2 147.1" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
   </g>
-  <g id="leg_l">
-    <path d="M 188.0 148.2 L 233.9 151.0" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
-    <path d="M 233.9 151.0 L 273.9 152.2" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
-    <path d="M 273.9 152.2 L 282.1 159.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+  <g id="leg_r">
+    <path d="M 193.9 147.8 L 239.9 147.9" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 239.9 147.9 L 279.9 146.8" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 279.9 146.8 L 290.0 151.1" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
   </g>
-  <path id="girdle" d="M 126.3 92.5 L 126.3 90.6 L 132.3 90.6" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
+  <path id="girdle" d="M 106.7 124.5 L 106.7 122.5 L 112.6 122.6" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
   <path id="pelvisBar" d="M 188.0 148.2 L 188.0 147.0 L 193.9 147.8" stroke="#0d9488" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
-  <path id="spine" d="M 188.0 147.0 Q 148.4 126.6 126.3 90.6" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
-  <g id="arm_l">
-    <path d="M 126.3 92.5 L 117.2 120.6" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
-    <path d="M 117.2 120.6 L 107.1 148.4" stroke="url(#grad-arm_l-1)" stroke-width="5.97101" stroke-linecap="round"/>
+  <path id="spine" d="M 188.0 147.0 Q 146.0 135.9 106.7 122.5" stroke="#0d9488" stroke-width="6" stroke-linecap="round" stroke-linejoin="round"/>
+  <g id="leg_l">
+    <path d="M 188.0 148.2 L 234.0 149.9" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+    <path d="M 234.0 149.9 L 274.0 150.2" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+    <path d="M 274.0 150.2 L 283.9 154.8" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
   </g>
-  <circle id="head" cx="116.6" cy="73.3" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
-  <line id="nose" x1="106.8" y1="75.2" x2="100.0" y2="76.5" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
+  <g id="arm_l">
+    <path d="M 106.7 124.5 L 82.5 141.8" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+    <path d="M 82.5 141.8 L 53.9 150.9" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+  </g>
+  <circle id="head" cx="89.1" cy="113.3" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
+  <line id="nose" x1="81.8" y1="120.2" x2="76.8" y2="125.0" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
   <g font-family="-apple-system, Helvetica, sans-serif" font-size="11" fill="#6b7180">
     <line x1="224" y1="16" x2="238" y2="16" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>
     <text x="243" y="20">near</text>