Library clips: Hips-only translations to avoid distorting the model

Some Mixamo clips carry translation keyframes on every bone; bound to a
model with different proportions they stretch/crush the limbs (bone
lengths must come from the target skeleton). Library binding now keeps
translations only for the Hips (root motion); rotations drive the rest.
Clips baked into a model keep their full channels.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/model3d.js

@@ -498,6 +498,10 @@ class ModelSim {
         for (const ch of clip.channels) {
           const ni = idxByBase[ch.bone];
           if (ni === undefined) continue;
+          // foreign clips: translations only for the Hips — per-bone
+          // translations carry the SOURCE rig's proportions and would
+          // stretch/crush a model with different bone lengths
+          if (ch.path === 'translation' && ch.bone !== 'Hips') continue;
           channels.push({ node: ni, path: ch.path, times: ch.times, vals: ch.vals });
         }
         if (channels.length > 4)