implement virtual CWD tracking and relative path resolution

[mavdol]

More control cwd control inside the sandbox:

process.chdir('hello-folder')   // _virtualCwd = 'hello-folder'
fs.readdirSync('.')             // '.' → 'hello-folder' → matches preopen 
fs.readFileSync('file.txt')     // 'file.txt' → 'hello-folder/file.txt'