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/Standing Side Bend/motion.json

@@ -6,7 +6,7 @@
   "frames": [
     {
       "hold": 0.5, "tween": 1.0,
-      "root": {"pos": [160, 72]},
+      "root": {"pos": [160, 64]},
       "spine": [{"lateral": 16}, {"lateral": 15}],
       "neck": 0,
       "shoulder_l": {"flexion": 14, "abduction": 126}, "elbow_l": 50,
@@ -17,7 +17,7 @@
     },
     {
       "hold": 0.3, "tween": 0.9,
-      "root": {"pos": [160, 72]},
+      "root": {"pos": [160, 64]},
       "spine": [{"lateral": 0}, {"lateral": 0}],
       "neck": 0,
       "shoulder_l": {"flexion": 4, "abduction": 8}, "elbow_l": 14,
@@ -28,7 +28,7 @@
     },
     {
       "hold": 0.5, "tween": 1.0,
-      "root": {"pos": [160, 72]},
+      "root": {"pos": [160, 64]},
       "spine": [{"lateral": -16}, {"lateral": -15}],
       "neck": 0,
       "shoulder_r": {"flexion": 14, "abduction": 126}, "elbow_r": 50,
@@ -39,7 +39,7 @@
     },
     {
       "hold": 0.3, "tween": 0.9,
-      "root": {"pos": [160, 72]},
+      "root": {"pos": [160, 64]},
       "spine": [{"lateral": 0}, {"lateral": 0}],
       "neck": 0,
       "shoulder_l": {"flexion": 4, "abduction": 8}, "elbow_l": 14,