This pull request strengthens the validation logic for blocks by enforcing and testing that the first transaction in the first subtree is a coinbase placeholder. It makes this check active in the production code and adds comprehensive tests to ensure correct and incorrect scenarios are handled as expected.

Validation logic improvements:

• Enforced the check in Block.Valid to require that the first transaction in the first subtree is a coinbase placeholder, returning an error if not.

Testing improvements:

• Added a new test TestBlock_Valid_CoinbasePlaceholderCheck in Block_test.go that covers:
  • Valid blocks with the coinbase placeholder in the correct position.
  • Invalid blocks with a regular transaction in the first position.
  • Invalid blocks with the coinbase placeholder in the wrong subtree.
  • Behavior when subtree slices are empty.