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/Calfs/frames/frame-1.svg

@@ -17,8 +17,8 @@
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   </g>
   <g id="arm_l">
-    <path d="M 134.1 35.2 L 161.9 46.3" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
-    <path d="M 161.9 46.3 L 187.8 31.5" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 134.1 35.2 L 142.2 63.7" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
+    <path d="M 142.2 63.7 L 170.0 74.8" stroke="#a9afba" stroke-width="5" stroke-linecap="round"/>
   </g>
   <path id="girdle" d="M 134.1 35.2 L 128.1 35.2 L 128.1 37.1" stroke="#3a3f4b" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>
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@@ -29,8 +29,8 @@
     <path d="M 212.0 148.8 L 218.7 140.2" stroke="url(#grad-leg_r-2)" stroke-width="5.98397" stroke-linecap="round"/>
   </g>
   <g id="arm_r">
-    <path d="M 128.1 37.1 L 155.7 48.8" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
-    <path d="M 155.7 48.8 L 183.3 37.4" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+    <path d="M 128.1 37.1 L 138.5 64.8" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
+    <path d="M 138.5 64.8 L 166.0 76.6" stroke="#3a3f4b" stroke-width="6" stroke-linecap="round"/>
   </g>
   <path d="M 174.9 117.9 L 202.9 117.9" stroke="#6b7180" stroke-width="7" stroke-linecap="round" stroke-linejoin="round"/>
   <path d="M 207.2 150.4 L 220.8 146.7" stroke="#6b7180" stroke-width="5" stroke-linecap="round" stroke-linejoin="round"/>