Zig 0.16 Migration — src/main.zig

• I/O types removed. std.fs.File.Writer and std.fs.File.Reader no longer exist as Zig 0.16 rewrote the
  entire I/O system around an Io event-loop abstraction. Rather than wiring up the new async-capable API, the
  fix uses libc directly: std.c.write() for stdout output and std.c.read() for stdin input, keyed on the
  POSIX file descriptor constants std.posix.STDOUT_FILENO / std.posix.STDIN_FILENO.

• std.io namespace removed. std.io.getStdOut().writer() and .reader() no longer exist; the global
  initialization in main() was simply deleted since we now use raw fds.

• std.fs.cwd().openFile() removed. The filesystem API also moved under the new Io abstraction. The
  lldb-fallback tty detection was rewritten using std.posix.openat() and std.c.close() to get a raw fd for
  the ioctl call.

• std.posix.getrandom() removed. Replaced with std.c.arc4random_buf(), which is natively available on
  macOS/Darwin.

• std.time.sleep() and std.time.milliTimestamp() removed. The std.time module was stripped down to
  constants only. Two small helpers were added: sleepNs() wrapping std.c.nanosleep(), and milliTimestamp()
  wrapping std.c.gettimeofday().

• std.fmt.fmtIntSizeBin() removed. The human-readable byte-size formatter was dropped from std.fmt.
  The FPS status line was updated to display raw byte counts instead.