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/Kneeling Hip Flexor Stretch/motion.json

@@ -6,23 +6,23 @@
     {
       "hold": 2.2, "tween": 1.8,
       "root": {"pos": [160, 108], "pitch": 4},
-      "spine": [11, 11],
+      "spine": [40, 40],
       "neck": 8, "head": -12,
-      "shoulder_r": 54, "elbow_r": 36,
-      "shoulder_l": 54, "elbow_l": 36,
+      "shoulder_r": 12, "elbow_r": 70,
+      "shoulder_l": 12, "elbow_l": 66,
       "hip_r": 86, "knee_r": 90, "ankle_r": 6,
-      "hip_l": -16, "knee_l": 74, "ankle_l": -55,
+      "hip_l": -16, "knee_l": 80, "ankle_l": -55,
       "pins": {"foot_r": [206, 150], "hand_r": [202, 112], "hand_l": [206, 114]}
     },
     {
       "hold": 2.4, "tween": 1.8,
       "root": {"pos": [162, 110], "pitch": 6},
-      "spine": [15, 15],
+      "spine": [42, 42],
       "neck": 9, "head": -13,
-      "shoulder_r": 58, "elbow_r": 32,
-      "shoulder_l": 58, "elbow_l": 32,
+      "shoulder_r": -2, "elbow_r": 92,
+      "shoulder_l": -2, "elbow_l": 88,
       "hip_r": 90, "knee_r": 90, "ankle_r": 6,
-      "hip_l": -28, "knee_l": 56, "ankle_l": -55,
+      "hip_l": -28, "knee_l": 62, "ankle_l": -55,
       "pins": {"foot_r": [210, 150], "hand_r": [206, 110], "hand_l": [210, 112]}
     }
   ]