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

Workouts/Resources/ExerciseMotions/Running.info.md

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+# Running
+
+Steady, easy-pace running — an upright posture with a slight forward lean from
+the ankles, a light midfoot landing under the hips, and a relaxed, efficient
+stride. Run in place, on a treadmill, or outdoors; your Apple Watch records
+the real effort — heart rate, calories, and duration — as a running workout.
+
+- **Category:** Cardio
+- **Type:** Aerobic / conditioning
+- **Targets:** Heart, lungs, legs, endurance
+- **Prescription:** 15–30 minutes at a conversational-to-brisk pace
+- **Defaults:** 1 × 1200 s
+
+## Setup
+
+Stand tall with a slight forward lean from the ankles, not the waist. Let
+your watch start the session so the effort is tracked on your wrist.
+
+## Execution
+
+1. Ease in for the first minute or two, letting your heart rate climb.
+2. Land lightly on your midfoot, under your hips — not out in front.
+3. Drive your knees forward and let your arms swing opposite your legs.
+4. Favor a quicker cadence over a longer stride; settle into a pace you can
+   hold for the whole block.
+5. Cool down with an easy minute before you stop.
+
+## Cues
+
+- Keep your posture tall, with the lean coming from the ankles.
+- Relax your shoulders and keep your elbows around 90 degrees.
+- Aim for a pace where you could speak in short sentences, not full paragraphs.
+
+## Common Mistakes
+
+- Overstriding — reaching the foot out ahead of the hips on landing.
+- Starting too hard and fading long before the time is up.
+- Hunching forward at the waist instead of leaning from the ankles.
+
+## Progression
+
+Add minutes, lift the pace, or fold in short faster intervals to raise the
+challenge over time.