Add chain validation and consensus mechanism

[devduart]

Summary

Implement functionality to verify blockchain integrity and allow choosing the longest valid chain in multi-node environments.

Tasks

• Create IsValid(chain Blockchain) bool to verify:
  • All blocks are correctly linked (PrevHash == previous.Hash)
  • All hashes are valid given the difficulty
• Implement a method to compare two chains and pick the longest valid one
• Optionally log validation errors for debugging

Notes

This will be used during peer-to-peer sync to determine which chain is canonical.