Add keyframe support to attach.

Limitations:
- Supports attaching a single keyframe. If multiple keyframes are present in the child, only the first one will be attached.
- Does not support time, ctrl, mpos, mquat.
- If mjs_detachBody is used, all keyframes are removed.
PiperOrigin-RevId: 662517666
Change-Id: If6e3839471e1884afc14e1a75e7ce1957c2f3581

src/user/user_api.cc

@@ -101,7 +101,7 @@ void mj_recompile(mjSpec* s, const mjVFS* vfs, mjModel* m, mjData* d) {
   modelC->Compile(vfs, &m);
   if (d) {
     modelC->MakeData(m, &d);
-    modelC->RestoreState(d->qpos, d->qvel, d->act);
+    modelC->RestoreState(m->qpos0, d->qpos, d->qvel, d->act);
     d->time = time;
   }
 }