fix(miner): use saturating_sub in bid gas-limit check

[mark0-cn]

Summary

The bid gas-limit guard in BidSimulator (src/node/miner/bid_simulator.rs) computed gas_limit - system_txs_gas - PAY_BID_TX_GAS_LIMIT with plain u64 subtraction. The sibling check ~80 lines below already uses saturating_sub for the same quantity, so this is an inconsistency.

If the reserved system-tx gas estimate plus PAY_BID_TX_GAS_LIMIT exceeds the block gas limit, the plain subtraction underflows — panicking in debug builds, and wrapping to a near-u64::MAX value in release, which inverts the check so the "gas limit exceeded" guard never fires. Mainnet gas limits are large so the trigger probability is low, but it's an unguarded arithmetic op on the MEV bid path fed by external builders.

Fix: use saturating_sub, matching the neighbouring code.

[hashdit-bot]

Pull Request Review

This PR fixes an arithmetic safety bug in the miner bid gas-limit guard by replacing plain u64 subtraction with chained saturating_sub in src/node/miner/bid_simulator.rs. The change aligns this guard with nearby existing logic and prevents underflow behavior that could otherwise panic in debug builds or wrap in release builds. Overall, it hardens bid validation on an externally influenced MEV path without changing business logic.

Sensitive Content

No sensitive content detected.

Security Issues

No serious security issues detected.

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[mark0-cn]

@joey0612