…ce, DifficultyEngine, and MerkleProofService

…module split

- Added ChainPersistence class for managing chunk storage and retrieval.
- Implemented save/load functionality for chunks, keys, streams, and stream indices.
- Introduced methods for chunk validation and recovery.
- Created DifficultyEngine class to calculate and adjust mining difficulty based on block timestamps.
- Added unit tests for ChainPersistence and DifficultyEngine to ensure functionality and correctness.
- Integrated MerkleProofService for generating and verifying Merkle proofs.
- Updated Blockchain class to utilize ChainPersistence and DifficultyEngine.
- Modified Makefile to include new source files and tests.

…nsistency