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/Bench Press/motion.json

@@ -18,10 +18,10 @@
       "neck": 8, "head": 0,
       "shoulder_r": 15, "elbow_r": 100,
       "shoulder_l": 15, "elbow_l": 100,
-      "hip_r": 78, "knee_r": 82, "ankle_r": 5,
-      "hip_l": 78, "knee_l": 82, "ankle_l": 5,
+      "hip_r": 10, "knee_r": 99, "ankle_r": 5,
+      "hip_l": 10, "knee_l": 102, "ankle_l": 5,
       "pins": {"foot_r": [196, 148], "foot_l": [200, 150],
-               "hand_r": [93, 100], "hand_l": [97, 102]}
+               "hand_r": [73, 98], "hand_l": [77, 100]}
     },
     {
       "hold": 0.4, "tween": 1.2,
@@ -29,9 +29,9 @@
       "spine": [0, 0],
       "neck": 8, "head": 0,
       "shoulder_r": 90, "elbow_r": 5,
-      "shoulder_l": 90, "elbow_l": 5,
-      "hip_r": 78, "knee_r": 82, "ankle_r": 5,
-      "hip_l": 78, "knee_l": 82, "ankle_l": 5,
+      "shoulder_l": {"flexion": 90, "rotation": -90}, "elbow_l": 5,
+      "hip_r": 10, "knee_r": 99, "ankle_r": 5,
+      "hip_l": 10, "knee_l": 102, "ankle_l": 5,
       "pins": {"foot_r": [196, 148], "foot_l": [200, 150],
                "hand_r": [67, 57], "hand_l": [71, 59]}
     }