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/Triceps Pushdown/motion.json

@@ -16,12 +16,12 @@
       "root": {"pos": [150, 66], "pitch": 8},
       "spine": [0, 2],
       "neck": 6, "head": -8,
-      "shoulder_r": 15, "elbow_r": 130,
-      "shoulder_l": 15, "elbow_l": 130,
+      "shoulder_r": -32, "elbow_r": 148,
+      "shoulder_l": -39, "elbow_l": 150,
       "hip_r": 6, "knee_r": 5, "ankle_r": 0,
       "hip_l": 6, "knee_l": 5, "ankle_l": 0,
-      "pins": {"foot_r": [146, 148], "foot_l": [150, 150],
-               "hand_r": [172, -10], "hand_l": [176, -8]}
+      "pins": {"foot_r": [146, 148], "foot_l": [150, 149],
+               "hand_r": [172, -4], "hand_l": [176, -2]}
     },
     {
       "hold": 0.4, "tween": 1.1,
@@ -32,7 +32,7 @@
       "shoulder_l": 8, "elbow_l": 10,
       "hip_r": 6, "knee_r": 5, "ankle_r": 0,
       "hip_l": 6, "knee_l": 5, "ankle_l": 0,
-      "pins": {"foot_r": [146, 148], "foot_l": [150, 150],
+      "pins": {"foot_r": [146, 148], "foot_l": [150, 149],
                "hand_r": [168, 40], "hand_l": [172, 42]}
     }
   ]