bug(EL): payload builder still panics on cumulative-gas checked_add overflow

[crazywriter1]

Bug

crates/execution-payload/src/payload.rs still panics on two cumulative-gas accounting paths via:

checked_add(...).expect("total gas shouldn't overflow")

1. Pre-execution capacity check (pool_tx.gas_limit())
2. Post-execution cumulative update (gas_used)

If either checked_add returns None, the payload builder aborts instead of failing with a structured PayloadBuilderError / invalid-tx path.

Why this is a bug (not a refactor)

• fix: use checked arithmetic for cumulative gas accounting in payload builder #24 asked for explicit error propagation instead of panic / silent clamp on this accounting.
• fix: use checked arithmetic for cumulative gas accounting in payload builder #42 switched to checked_add but left both .expect(...) sites in place, so the panic path remains.
• Closed PR fix: complete payload builder gas overflow error propagation #135 proposed finishing that fix; it was closed for process (Closes: #… / assignment), not because the change was rejected on technical grounds.

Impact

In production the overflow is extremely unlikely (gas_used / gas_limit are bounded by the block gas limit, far below u64::MAX). The bug is still real: an unreachable panic is the wrong failure mode for block building, and it is inconsistent with the error-propagation pattern already required for this code.

Proposed fix

• Pre-execution: treat add overflow like “does not fit” (ExceedsGasLimit / mark invalid), do not panic.
• Post-execution: return PayloadBuilderError on overflow and fail the build cleanly.

No hardfork gate: builder-local error handling only.

Acceptance criteria

• Both .expect("total gas shouldn't overflow") call sites in payload.rs are removed
• Overflow / non-fit paths return errors (no panic)
• cargo test -p arc-execution-payload passes

[crazywriter1]

Hello. If this looks useful, could I please be assigned so I can open a PR with Closes: #<this> once you're ready?

I'd keep the first PR limited to the two payload.rs sites (pre-check → mark invalid / ExceedsGasLimit; post-exec → PayloadBuilderError). Happy to leave the executor path for a separate follow-up if you prefer. @ZhiyuCircle