Remember compiler options during attach.

Note:
- Move compiler options into `mjsCompiler` struct.
- The compiler options from the attached model are not written by mj_SaveXML.
PiperOrigin-RevId: 689391440
Change-Id: I1d63c146a32f87c737b7a55b64a54b0ffe3aecc9

src/user/user_objects.h

@@ -212,7 +212,8 @@ class mjCBase : public mjCBase_ {
   mjCBase& operator=(const mjCBase& other);
 
   mjCFrame* frame;                // pointer to frame transformation
-  mjCModel* model;                // pointer to model that created object
+  mjCModel* model;                // pointer to model that owns object
+  mjsCompiler* compiler;          // pointer to the compiler options
 
   virtual ~mjCBase() = default;   // destructor