Resize keyframes also at the start of compilation.

Previously, it was assumed that attaching was the last operation performed on the mjSpec, so keyframes resulted in the incorrect size. In order to prevent information loss, we now only allow the resize function to expand the keyframe array, since attach should never remove a degree of freedom.

PiperOrigin-RevId: 804917828
Change-Id: Icfacd207ced1c6aac600d514f8d9065ceff2d347

src/user/user_model.h

@@ -432,8 +432,8 @@ class mjCModel : public mjCModel_, private mjSpec {
   // convert pending keyframes info to actual keyframes
   void ResolveKeyframes(const mjModel* m);
 
-  // resize a keyframe, filling in missing values
-  void ResizeKeyframe(mjCKey* key, const mjtNum* qpos0_, const mjtNum* bpos, const mjtNum* bquat);
+  // expand a keyframe, filling in missing values
+  void ExpandKeyframe(mjCKey* key, const mjtNum* qpos0_, const mjtNum* bpos, const mjtNum* bquat);
 
   // compute qpos0
   void ComputeReference();
@@ -463,6 +463,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   // delete all plugins created by the subtree
   void DeleteSubtreePlugin(mjCBody* subtree);
 
+  // expand all keyframes in the model
+  void ExpandAllKeyframes();
+
   mjListKeyMap ids;   // map from object names to ids
   mjCError errInfo;   // last error info
   std::vector<mjKeyInfo> key_pending_;  // attached keyframes