feat: Add Rex hardfork support

[flyq]

Summary

This PR introduces support for the Rex hardfork (the second MegaETH hardfork) in the stateless validator and updates the test data to reflect the new chain configuration.

Changes

Rex Hardfork Implementation (validator-core/src/chain_spec.rs)

• Added new Rex variant to the MegaethHardfork enum
• Implemented is_rex_active_at_timestamp() method in the MegaethHardforks trait
• Added rex_time field to MegaethGenesisHardforks for genesis configuration
• Updated spec_id_at_timestamp() to return SpecId::REX when the Rex hardfork is active
• Updated MEGA_MAINNET_HARDFORKS static config to include Rex hardfork (activated at timestamp 0)
• Added Debug derive to ChainSpec struct for improved debugging

Test Data Updates

• Updated genesis/genesis-6342.json with Rex hardfork configuration
• Replaced old block test data with new blocks (269-290) that include Rex hardfork support
• Updated witness files (.mpt and .salt) for the new test blocks
• Updated test_data/contracts.txt with new contract code hash

Hardfork Priority

The hardfork activation now follows this priority order:

1. Rex (newest) → SpecId::REX
2. MiniRex → SpecId::MINI_REX
3. Equivalence (default) → SpecId::EQUIVALENCE