Function setFeeIncentiveSubsidyRate

Set the fraction of each mint's trading fee paid from the market's sponsor-funded fee-incentive balance. Read live at mint time, so the new rate applies to the next mint on every market, including markets already trading; 0 stops incentives from being spent without moving them. The subsidy never changes the trading fee charged, only how much of it the trader pays; on a referred mint the referral is computed on the trader-paid part, so a higher rate also shrinks the referral.

Binds every mint only once the version watermark has retired package versions older than 4: those compiled in a fixed 20% and never read this rate, so until then a mint routed through one still draws 20% from the market's balance. A zero rate does not stop rebalance_expiry_cash allocating the pool reserve into markets; to wind incentives down, also withdraw the reserve (plp::withdraw_fee_incentives), or, once the watermark has retired versions older than 4, set the live target rate to zero, which keeps the reserve in the pool.

Not gated on the valuation flag, matching set_referral_fee_rate: nothing in the flush reads this rate, and a mint that consumes a subsidy mid-flush lands after the snapshot captured that market's cash, so it cannot reach the frozen mark.