fix(demo): correct directory rename operation in DemonstrateVFS

[phmatray]

Fixes #130

Problem

The RenameDirectory demo step was commented out with a TODO because it was using the wrong method signature. The method signature is RenameDirectory(VFSDirectoryPath, string), not RenameDirectory(VFSDirectoryPath, VFSDirectoryPath).

Solution

• Fixed the second parameter to be a string ("heroes") instead of a VFSDirectoryPath
• Uncommented the directory rename step
• Removed the TODO comment

Testing

✅ All 379 unit tests pass on net10.0
✅ All 71 GitHub tests pass on net10.0
✅ Build succeeds with 0 errors

Changes

// Before (commented out):
// ProcessStep(vfs, "RENAME DIRECTORY",
//     () => vfs.RenameDirectory(new VFSDirectoryPath("/avengers"), new VFSDirectoryPath("/heroes")));

// After (fixed):
ProcessStep(vfs, "RENAME DIRECTORY",
    () => vfs.RenameDirectory(new VFSDirectoryPath("/avengers"), "heroes"));

The directory rename operation now works correctly in the demo CLI.