Add Ethereum finality logic: Casper FFG epoch tracking

[geminimir]

Summary

Implement Ethereum-specific finality strategy: track block confirmations and Casper FFG finality (~12-15 minutes, 2 epochs).

Changes

• Track confirmation count via newHeads subscription
• Emit chain.tx.confirming at thresholds: 1, 6, 12 confirmations
• Mark finalized when tx block is finalized by Casper FFG (eth_getBlockByNumber with finalized tag)

Tests

• Confirmations increment correctly with new blocks
• Finalized tag match triggers finalized state

Acceptance Criteria

Ethereum finality tracking reflects PoS finality guarantees.