Controller-aware wrappers around :atomics.

Each atomic operation is itself atomic (as the name implies), but compositions of atomics aren't. For example, read; modify; write via :atomics.get/2 + :atomics.put/3 is racy across processes. Lockstep wraps each call as a sync point so the scheduler can interleave between the get and the put, surfacing such races.

When called outside a Lockstep test (no controller in process dictionary), the wrapper falls through to the underlying atomic call without yielding -- so the same code runs identically in production and under controlled scheduling.