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

Workouts/Resources/ExerciseMotions/Dumbbell Fly.motion.json

@@ -20,9 +20,9 @@
       "neck": 8, "head": 0,
       "shoulder_r": 88, "elbow_r": 12,
       "shoulder_l": 88, "elbow_l": 12,
-      "hip_r": {"flexion": 78, "abduction": 10}, "knee_r": 82, "ankle_r": 5,
-      "hip_l": {"flexion": 78, "abduction": 10}, "knee_l": 82, "ankle_l": 5,
-      "pins": {"foot_r": [142, 148], "foot_l": [178, 150]}
+      "hip_r": {"flexion": 10, "abduction": 15}, "knee_r": 93, "ankle_r": 5,
+      "hip_l": {"flexion": 7, "abduction": 16}, "knee_l": 91, "ankle_l": 5,
+      "pins": {"foot_r": [145, 148], "foot_l": [176, 150]}
     },
     {
       "hold": 0.5, "tween": 1.3,
@@ -31,9 +31,9 @@
       "neck": 8, "head": 0,
       "shoulder_r": {"flexion": 16, "abduction": 84}, "elbow_r": 22,
       "shoulder_l": {"flexion": 16, "abduction": 84}, "elbow_l": 22,
-      "hip_r": {"flexion": 78, "abduction": 10}, "knee_r": 82, "ankle_r": 5,
-      "hip_l": {"flexion": 78, "abduction": 10}, "knee_l": 82, "ankle_l": 5,
-      "pins": {"foot_r": [142, 148], "foot_l": [178, 150]}
+      "hip_r": {"flexion": 10, "abduction": 15}, "knee_r": 93, "ankle_r": 5,
+      "hip_l": {"flexion": 7, "abduction": 16}, "knee_l": 91, "ankle_l": 5,
+      "pins": {"foot_r": [145, 148], "foot_l": [176, 150]}
     }
   ]
 }