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/Side Plank/frames/frame-2.svg

@@ -4,11 +4,11 @@
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-  <path d="M 48.0 160.9 L 277.9 160.9 L 277.9 150.5 L 48.0 150.5 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
+  <path d="M 48.0 160.9 L 279.4 160.9 L 279.4 150.5 L 48.0 150.5 Z" fill="none" stroke="#b9bec9" stroke-width="3" stroke-linejoin="round"/>
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@@ -19,8 +19,8 @@
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