Refactor: promote BlockchainHeaderValidator to lib/

[mw2000]

Summary

EEVM.TestSupport.BlockchainHeaderValidator is consensus logic (parent linkage, number, timestamp, extra-data, gas-limit elasticity) that lives under test/support/ only because EEVM.Block.Header doesn't carry the fields it needs.

Scope

• Expand EEVM.Block.Header with extra_data, gas_used, hash, and difficulty fields (and any other fields the validator needs).
• Move the validator to lib/eevm/block/header_validator.ex, retyped against Block.Header.t() rather than the fixture struct.
• Update the BlockchainTest runner to construct a real Block.Header from fixtures and call the lib-side validator.

Definition of Done

• Validator lives in lib/.
• BlockchainTests still pass at the same coverage level.
• No remaining test-support shims for header fields.

Notes

• Came out of feat(test): add BlockchainTest harness (ValidBlocks + InvalidBlocks + TransitionTests) #118.