Prove the visibility bug

Implement static int SpinUntilFlag(int ms) that busy-waits (Thread.Sleep(1) loop, bounded by ms) until a static bool Flag set by static void SetFlag() becomes true, returning 1 if seen, 0 on timeout — but read Flag through Volatile.Read so the JIT cannot cache it in a register. The test sets the flag from another thread mid-spin and expects success.