Add a max-fee-per-gas headroom factor to the driver gas config

[squadgazzz]

Description

When mainnet base fee jumps between the moment a settlement is submitted and the moment it would be included, the transaction can end up stranded below the new base fee and expires unmined. We saw a burst of these during the Sep 4 gas spike (0.7 to ~4.75 gwei in a few minutes): valid solutions that never landed because their max_fee_per_gas was set from a pre-spike estimate.

Today the driver submits with exactly the estimator's max_fee_per_gas (only raised to cover the tip, then capped at gas-price-cap), with no headroom. This adds a configurable factor on top of the estimate.

max_fee_per_gas is a ceiling, not the amount paid: a settlement always pays the real base_fee + priority_fee. So extra headroom changes nothing in calm conditions and only bites when base fee rises after submission. Unlike raising the priority fee, it does not add per-settlement cost, and it is a separate lever from the tip aimed at the expiry case.

The default is 1.0, so behavior is unchanged until a network opts in via config.

Changes

• Added a max-fee-per-gas-factor option to the driver [submission] config, defaulting to 1.0 (no headroom)
• Applied the factor to the estimated max_fee_per_gas before the gas-price cap check, in the driver's gas price estimator

How to test

Existing tests. The default of 1.0 is a no-op; set it above 1.0 for a network and confirm submitted transactions carry a proportionally higher max_fee_per_gas while the effective price paid still tracks base fee plus tip.