Reprice ModExp precompile (EIP-7883)

[mw2000]

Summary

Increase the gas cost of the ModExp precompile (address 0x05). Activates with Fusaka.

Spec

• EIP-7883: ModExp Gas Cost Increase — https://eips.ethereum.org/EIPS/eip-7883

Scope

• Add Fusaka hardfork + EIP-7883 flag to EEVM.HardforkConfig.
• Branch EEVM.Precompiles.ModExp gas formula on the flag.
• Cross-check with EIP-7823 input bounds (separate issue).

Definition of Done

• ModExp gas formula returns the new cost when Fusaka is active and the legacy cost otherwise.
• Fusaka ModExp StateTest fixtures pass via the harness.

Parent

• EVM Completeness: Missing opcodes, precompiles, and gas rules #30 — EVM Completeness tracking issue