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/Plank.motion.json

@@ -10,18 +10,18 @@
       "neck": 0, "head": -65,
       "shoulder_r": 95, "elbow_r": 90,
       "shoulder_l": 95, "elbow_l": 90,
-      "hip_r": 60, "knee_r": 55,
-      "hip_l": 60, "knee_l": 55
+      "hip_r": 60, "knee_r": 72,
+      "hip_l": 60, "knee_l": 72
     },
     {
       "hold": 1.8, "tween": 0.7,
-      "root": {"pos": [168, 137], "yaw": 180, "pitch": 80},
+      "root": {"pos": [168, 136], "yaw": 180, "pitch": 80},
       "spine": [0, 0],
       "neck": 0, "head": -50,
-      "shoulder_r": 80, "elbow_r": 90,
-      "shoulder_l": 80, "elbow_l": 90,
-      "hip_r": 0, "knee_r": 0,
-      "hip_l": 0, "knee_l": 0
+      "shoulder_r": 82, "elbow_r": 90,
+      "shoulder_l": 82, "elbow_l": 90,
+      "hip_r": 0, "knee_r": 14, "ankle_r": -50,
+      "hip_l": 0, "knee_l": 14, "ankle_l": -50
     }
   ]
 }