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/Goblet Squat/motion.json

@@ -16,7 +16,7 @@
       "shoulder_l": 5, "elbow_l": 140,
       "hip_r": 5, "knee_r": 4, "ankle_r": 0,
       "hip_l": 5, "knee_l": 4, "ankle_l": 0,
-      "pins": {"foot_r": [162, 148], "foot_l": [166, 150],
+      "pins": {"foot_r": [162, 148], "foot_l": [166, 147],
                "hand_r": [185, -8], "hand_l": [189, -6]}
     },
     {
@@ -28,7 +28,7 @@
       "shoulder_l": 15, "elbow_l": 135,
       "hip_r": 96, "knee_r": 116, "ankle_r": 20,
       "hip_l": 96, "knee_l": 116, "ankle_l": 20,
-      "pins": {"foot_r": [162, 148], "foot_l": [166, 150],
+      "pins": {"foot_r": [162, 148], "foot_l": [166, 147],
                "hand_r": [196, 36], "hand_l": [200, 38]}
     }
   ]