feat: Add atomicity and new getters to SafeBox

[harrytmthy]

Summary

Introduce atomic single-instance semantics and ergonomic accessors. SafeBox.get(...) and SafeBox.getOrNull(...) provide singleton-style retrieval.

Implementation Details

• Added get(fileName) (throws if not initialized) and getOrNull(fileName).
• First hit uses a synchronized double-check on the instances map to serialize construction.

Closes #64