fix: use checked arithmetic for cumulative gas accounting in payload builder

[Himess]

Summary

Use checked arithmetic for cumulative gas accounting in the payload builder to prevent potential overflow on u64 addition.

Two instances in arc_ethereum_payload:

• Line 579: cumulative_gas_used + pool_tx.gas_limit() — replaced with checked_add + is_none_or to reject the transaction if the addition overflows
• Line 660: cumulative_gas_used += gas_used — replaced with saturating_add to prevent wrapping

This follows the same defensive arithmetic pattern applied to reward_beneficiary in #21, where u128 multiplication was replaced with U256 arithmetic to prevent overflow.

[atiwari-circle]

Overflow here is not practically reachable -- cumulative_gas_used and pool_tx.gas_limit() are both bounded by block_gas_limit (~30M), nowhere near u64::MAX (~18.4 exagas). The checked_add at line 579 is fine if you want to keep it as defensive hardening, but saturating_add at line 660 is the wrong choice -- it would silently clamp at u64::MAX instead of failing the block build. Use checked_add with error propagation there, or leave it as plain addition.

[Himess]

Thanks for the review @atiwari-circle. Agreed, replaced saturating_add with checked_add + PayloadBuilderError propagation in 4755edd, so an overflow (still unreachable in practice) would now fail the build cleanly instead of silently clamping. Kept checked_add at line 579 as defensive hardening.