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/Standing Calf Stretch/motion.json

@@ -6,26 +6,27 @@
   "frames": [
     {
       "hold": 2.2, "tween": 1.8,
-      "root": {"pos": [160, 66], "pitch": 12},
-      "spine": [4, 4],
-      "neck": 6, "head": -10,
-      "shoulder_r": 24, "elbow_r": 42,
-      "shoulder_l": 24, "elbow_l": 42,
-      "hip_r": 22, "knee_r": 22, "ankle_r": 6,
-      "knee_l": 2, "ankle_l": 10,
-      "pins": {"foot_r": [188, 150], "foot_l": [142, 150]}
+      "root": {"pos": [158, 70], "pitch": 30},
+      "spine": [6, 6],
+      "neck": 6, "head": -12,
+      "shoulder_r": {"flexion": 39, "abduction": 5, "rotation": 6}, "elbow_r": 64,
+      "shoulder_l": {"flexion": 36, "abduction": 4, "rotation": 6}, "elbow_l": 58,
+      "hip_r": 30, "knee_r": 34, "ankle_r": 6,
+      "hip_l": 5, "knee_l": 2, "ankle_l": 12,
+      "pins": {"foot_r": [178, 150], "foot_l": [128, 150],
+               "hand_r": [236, 48], "hand_l": [238, 54]}
     },
     {
       "hold": 2.4, "tween": 1.8,
-      "root": {"pos": [160, 68], "pitch": 21},
-      "spine": [6, 6],
-      "neck": 9, "head": -17,
-      "shoulder_r": 40, "elbow_r": 34,
-      "shoulder_l": 40, "elbow_l": 34,
-      "hip_r": 44, "knee_r": 46, "ankle_r": 8,
-      "knee_l": 2, "ankle_l": 12,
-      "pins": {"foot_r": [198, 150], "foot_l": [132, 150],
-               "hand_r": [186, 110], "hand_l": [190, 112]}
+      "root": {"pos": [160, 71], "pitch": 36},
+      "spine": [7, 7],
+      "neck": 7, "head": -13,
+      "shoulder_r": {"flexion": 23, "abduction": -1, "rotation": 13}, "elbow_r": 100,
+      "shoulder_l": {"flexion": 20, "abduction": -4, "rotation": 15}, "elbow_l": 93,
+      "hip_r": 38, "knee_r": 40, "ankle_r": 6,
+      "hip_l": 5, "knee_l": 2, "ankle_l": 15,
+      "pins": {"foot_r": [181, 150], "foot_l": [128, 150],
+               "hand_r": [236, 48], "hand_l": [238, 54]}
     }
   ]
 }