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/Leg Swings/motion.json

@@ -5,8 +5,8 @@
   "working": ["leg_r"],
   "frames": [
     {
-      "hold": 0.16, "tween": 0.72,
-      "root": {"pos": [156, 66], "pitch": 5},
+      "hold": 0.16, "tween": 0.83,
+      "root": {"pos": [156, 64], "pitch": 5},
       "spine": [0, 0],
       "neck": 2, "head": -6,
       "shoulder_r": 6, "elbow_r": 10,
@@ -16,8 +16,8 @@
       "pins": {"foot_l": [168, 150]}
     },
     {
-      "hold": 0.16, "tween": 0.72,
-      "root": {"pos": [156, 66], "pitch": 5},
+      "hold": 0.16, "tween": 0.83,
+      "root": {"pos": [156, 64], "pitch": 5},
       "spine": [0, 0],
       "neck": 2, "head": -6,
       "shoulder_r": 6, "elbow_r": 10,