refactor(crypto): Rename SafeBoxExecutor to CipherPoolExecutor

[harrytmthy]

Summary

Renamed SafeBoxExecutor to CipherPoolExecutor to better reflect its true responsibility and prevent confusion across SafeBox internals. This is in preparation for #42 (LRU trimming), where CipherPoolExecutor will also handle scheduled trimming tasks. The renaming clarifies that the executor is not meant for general SafeBox tasks.

Implementation Details

• Renamed SafeBoxExecutor → CipherPoolExecutor
• Moved it into the same file as CipherPool.kt to reflect ownership
• Added CipherPoolExecutor as a constructor argument to CipherPool
• Introduced CipherPoolExecutor interface with executeLoadTask(...) and scheduleTrimmingTask(...)
• Provided DefaultCipherPoolExecutor singleton as the default implementation
• Updated CipherPool internals to delegate refill tasks via the executor

Closes #49