fix(evm): don't pay the beneficiary for the unused EIP-8037 reservoir

[Xowiek]

ArcEvmHandler::reward_beneficiary pays the beneficiary for:

let gas_used = exec_result.gas().used();

Under EIP-8037 that also counts gas the caller gets back. last_frame_result rebuilds the top-level gas over the whole tx gas limit, returns only the regular remaining, and keeps the unused state gas reservoir separately:

*gas = Gas::new_spent(evm.ctx().tx().gas_limit());
gas.erase_cost(remaining);
gas.set_reservoir(reservoir);

so used() — tx.gas_limit - remaining - refunded — still includes the reservoir. Arc doesn't override reimburse_caller, and revm's version hands remaining + reservoir + refunded back to the caller. The reservoir ends up both refunded and paid to the beneficiary.

revm's own post_execution::reward_beneficiary leaves it out for exactly this reason:

// Exclude reservoir gas (EIP-8037) from the used gas — reservoir is unused and reimbursed.
let effective_used = gas.used().saturating_sub(gas.reservoir());

EIP-8037 support landed in revm-handler 18.0.0. The Arc override was written before that and didn't pick the subtraction up when revm was bumped.

With a 20M gas limit, 1M regular gas left, a 3M reservoir and a price of 10, the caller is charged 200,000,000. On main, reimbursement plus reward come to:

  left: 230000000
 right: 200000000

The extra 30,000,000 is reservoir × price of native coin that was never debited from anyone.

This isn't reachable on any network today. The reservoir is only non-zero with EIP-8037 enabled, revm enables it at SpecId::AMSTERDAM, and no Arc chain spec schedules Amsterdam — mainnet and localdev stop at Osaka, testnet and devnet at Prague. So the change is a no-op for every existing block. create_evm already turns off revm's Amsterdam EIP-7708 logs ahead of the spec being enabled; this is the same kind of change for the fee path.

The added test runs revm's reimburse_caller and Arc's reward_beneficiary on the gas last_frame_result produces, and checks the two credits equal what the caller was charged. It fails on main with the output above. cargo test -p arc-evm passes (150 tests), as do fmt and clippy.