Implement `File.stat` method that performs an asynchronous `fstat` call on the open file descriptor, returning a `Stat` object populated with file metadata (device, inode, mode, link count, user, group, size, block size, block count). Refactor the existing `statPathCallback` into a generic `statCallback` shared by both `statPath` and the new `fileStat` function. Bump `MAX_METHODS_PER_CLASS` from 11 to 12 to accommodate the new foreign method slot. Add comprehensive test coverage including successful stat retrieval and error handling when called on a closed file.
