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/Dumbbell Row/motion.json

@@ -18,7 +18,7 @@
       "neck": 10, "head": -50,
       "shoulder_r": 75, "elbow_r": 8,
       "shoulder_l": 72, "elbow_l": 5,
-      "hip_r": 25, "knee_r": 25, "ankle_r": 10,
+      "hip_r": 96, "knee_r": 76, "ankle_r": 30,
       "hip_l": 80, "knee_l": 95, "ankle_l": -50,
       "pins": {"foot_r": [196, 148], "foot_l": [202, 122],
                "hand_l": [66, 120], "hand_r": [92, 125]}
@@ -28,9 +28,9 @@
       "root": {"pos": [168, 82], "yaw": 180, "pitch": 80},
       "spine": [0, 0],
       "neck": 10, "head": -50,
-      "shoulder_r": 40, "elbow_r": 115,
+      "shoulder_r": -21, "elbow_r": 115,
       "shoulder_l": 72, "elbow_l": 5,
-      "hip_r": 25, "knee_r": 25, "ankle_r": 10,
+      "hip_r": 96, "knee_r": 76, "ankle_r": 30,
       "hip_l": 80, "knee_l": 95, "ankle_l": -50,
       "pins": {"foot_r": [196, 148], "foot_l": [202, 122],
                "hand_l": [66, 120], "hand_r": [100, 86]}