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/Calfs.motion.json

@@ -22,8 +22,8 @@
       "neck": 16,
       "shoulder_r": 44, "elbow_r": 25,
       "shoulder_l": 44, "elbow_l": 25,
-      "hip_r": 62, "knee_r": -10, "ankle_r": 12,
-      "hip_l": 62, "knee_l": -10, "ankle_l": 12,
+      "hip_r": 62, "knee_r": 50, "ankle_r": 12,
+      "hip_l": 62, "knee_l": 56, "ankle_l": 12,
       "pins": {"hand_r": [166, 74], "hand_l": [170, 76], "foot_r": [212, 148], "foot_l": [216, 150]}
     },
     {
@@ -33,8 +33,8 @@
       "neck": 16,
       "shoulder_r": 44, "elbow_r": 25,
       "shoulder_l": 44, "elbow_l": 25,
-      "hip_r": 66, "knee_r": -8, "ankle_r": -18,
-      "hip_l": 66, "knee_l": -8, "ankle_l": -18,
+      "hip_r": 66, "knee_r": 59, "ankle_r": -18,
+      "hip_l": 66, "knee_l": 64, "ankle_l": -18,
       "pins": {"hand_r": [166, 74], "hand_l": [170, 76], "foot_r": [212, 139], "foot_l": [216, 141]}
     }
   ]