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/Romanian Deadlift/motion.json

@@ -9,27 +9,25 @@
   "frames": [
     {
       "hold": 0.4, "tween": 0.9,
-      "root": {"pos": [90, 92], "pitch": 68},
+      "root": {"pos": [100, 84], "pitch": 70},
       "spine": [0, -6],
-      "neck": 5, "head": -62,
-      "shoulder_r": 66, "elbow_r": 4,
-      "shoulder_l": 66, "elbow_l": 4,
-      "hip_r": 112, "knee_r": 12, "ankle_r": 12,
-      "hip_l": 112, "knee_l": 12, "ankle_l": 12,
-      "pins": {"foot_r": [158, 147], "foot_l": [162, 149],
-               "hand_r": [167, 120], "hand_l": [171, 122]}
+      "neck": 5, "head": -66,
+      "shoulder_r": 35, "elbow_r": 8,
+      "shoulder_l": 35, "elbow_l": 8,
+      "hip_r": 116, "knee_r": 18, "ankle_r": 12,
+      "hip_l": 116, "knee_l": 18, "ankle_l": 12,
+      "pins": {"foot_r": [156, 148], "foot_l": [160, 149]}
     },
     {
       "hold": 0.4, "tween": 1.4,
-      "root": {"pos": [158, 64], "pitch": 10},
+      "root": {"pos": [158, 64], "pitch": 8},
       "spine": [0, 0],
       "neck": 2, "head": -8,
-      "shoulder_r": 10, "elbow_r": 3,
-      "shoulder_l": 10, "elbow_l": 3,
-      "hip_r": 8, "knee_r": 8, "ankle_r": 0,
-      "hip_l": 8, "knee_l": 8, "ankle_l": 0,
-      "pins": {"foot_r": [158, 147], "foot_l": [162, 149],
-               "hand_r": [169, 38], "hand_l": [173, 40]}
+      "shoulder_r": -6, "elbow_r": 8,
+      "shoulder_l": -6, "elbow_l": 8,
+      "hip_r": 6, "knee_r": 10, "ankle_r": 0,
+      "hip_l": 6, "knee_l": 10, "ankle_l": 0,
+      "pins": {"foot_r": [156, 148], "foot_l": [160, 149]}
     }
   ]
 }