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

unity/Runtime/Bindings/MjBindings.cs

@@ -5716,6 +5716,26 @@ public unsafe struct mjrContext_ {
   public int readDepthMap;
 }
 
+[StructLayout(LayoutKind.Sequential)]
+public unsafe struct mjsCompiler_ {
+  public byte autolimits;
+  public double boundmass;
+  public double boundinertia;
+  public double settotalmass;
+  public byte balanceinertia;
+  public byte fitaabb;
+  public byte degree;
+  public fixed sbyte eulerseq[3];
+  public byte discardvisual;
+  public byte convexhull;
+  public byte usethread;
+  public byte fusestatic;
+  public int inertiafromgeom;
+  public fixed int inertiagrouprange[2];
+  public int alignfree;
+  public mjLROpt_ LRopt;
+}
+
 [StructLayout(LayoutKind.Sequential)]
 public unsafe struct mjuiState_ {
   public int nrect;