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/Crunch/frames/frame-1.svg

@@ -6,8 +6,8 @@
   </defs>
   <path d="M 39.4 161.1 L 231.6 161.1 L 231.6 150.6 L 39.4 150.6 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
   <g id="arm_l">
-    <path d="M 86.9 146.1 L 59.7 133.6" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
-    <path d="M 59.7 133.6 L 72.4 160.3" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 86.9 146.1 L 69.7 121.9" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 69.7 121.9 L 59.4 149.6" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
   </g>
   <g id="leg_l">
     <path d="M 171.9 146.8 L 199.5 110.5" stroke="url(#grad-leg_l-0)" stroke-width="5.12442" stroke-linecap="round"/>
@@ -23,8 +23,8 @@
     <path d="M 205.0 149.2 L 215.3 145.3" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
   </g>
   <g id="arm_r">
-    <path d="M 81.0 147.9 L 53.8 135.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
-    <path d="M 53.8 135.4 L 66.5 162.2" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+    <path d="M 81.0 147.9 L 63.8 123.7" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+    <path d="M 63.8 123.7 L 53.5 151.5" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
   </g>
   <circle id="head" cx="61.4" cy="141.9" r="10" fill="white" stroke="#3a3f4b" stroke-width="6"/>
   <line id="nose" x1="63.5" y1="132.1" x2="65.0" y2="125.4" stroke="#3a3f4b" stroke-width="4" stroke-linecap="round"/>

Exercise Library/Crunch/motion.json

@@ -8,8 +8,8 @@
       "root": {"pos": [166, 146], "pitch": -90},
       "spine": [0, 0],
       "neck": 12, "head": 0,
-      "shoulder_r": 155, "elbow_r": 140,
-      "shoulder_l": 155, "elbow_l": 140,
+      "shoulder_r": 125, "elbow_r": 125,
+      "shoulder_l": 125, "elbow_l": 125,
       "hip_r": 75, "knee_r": 85, "ankle_r": 5,
       "hip_l": 75, "knee_l": 85, "ankle_l": 5,
       "pins": {"foot_r": [205, 148], "foot_l": [209, 150]}