Re-implement VFS internals in C++, removing the constraints of the previous implementation.

PiperOrigin-RevId: 649433087
Change-Id: Icaa3e6b7f2ef14f56b04fef2f4360b46d8b5f022

unity/Runtime/Bindings/MjBindings.cs

@@ -41,8 +41,6 @@ public const double mjMINIMP = 0.0001;
 public const double mjMAXIMP = 0.9999;
 public const int mjMAXCONPAIR = 50;
 public const int mjMAXTREEDEPTH = 50;
-public const int mjMAXVFS = 2000;
-public const int mjMAXVFSNAME = 1000;
 public const int mjNEQDATA = 11;
 public const int mjNDYN = 10;
 public const int mjNGAIN = 10;
@@ -4978,11 +4976,7 @@ public unsafe struct mjLROpt_ {
 [StructLayout(LayoutKind.Sequential)]
 public unsafe struct _mjVFS
 {
-  public int nfile;
-  [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2000 * 1000)] public char[] filename;
-  [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2000)] public UIntPtr[] filesize;
-  [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2000)] public IntPtr[] filedata;
-  [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2000)] public UInt64[] filestamp;
+  public void* impl_;
 }
 
 [StructLayout(LayoutKind.Sequential)]

unity/Runtime/Tools/MjVfs.cs

@@ -40,11 +40,6 @@ public sealed class MjVfs : IDisposable {
     }
   }
 
-  // Number of files added to the filesystem.
-  public int FilesCount {
-    get { return Data.nfile; }
-  }
-
   // Adds a new file to the virtual filesystem.
   public unsafe void AddFile(string filename, string contents) {
     var contents_bytes = Encoding.UTF8.GetBytes(contents);

unity/Tests/Editor/Tools/MjVFSEditorTests.cs

@@ -40,7 +40,7 @@ namespace Mujoco {
       var filename = "filename";
       var contents = "contents";
       _vfs.AddFile(filename, contents);
-      Assert.That(_vfs.FilesCount, Is.EqualTo(1));
+      Assert.That(() => { _vfs.AddFile(filename, contents); }, Throws.Exception);  // duplicate file
     }
   }
 }