test: comprehensive integration tests for cross-backend operations

Added extensive integration tests to verify self-encryption functionality
across different storage backends:

* Added StorageBackend helper struct for managing memory/disk storage
* Added debug helpers for storage state visualization
* Implemented cross-backend tests:
  - Memory-to-memory, memory-to-disk, disk-to-memory operations
  - Large file handling (100MB+)
  - Concurrent access with multiple file sizes
  - Platform-specific size handling (page sizes, u16/u32 boundaries)
  - Error handling and recovery
* Added verification steps between operations
* Fixed chunk handling to ensure proper storage/retrieval flow
* Added detailed logging for debugging storage operations

These tests ensure consistent behavior across different storage backends
and verify data integrity through the entire encrypt/decrypt cycle.