Inspect production invocation spend, active reservations, and remaining headroom

Separates durable production invocation spend from admission-relevant reservations. Headroom subtracts both the applicable spent amount and active reservation amount. An unbacked reservation contributes its full unexpired amount; a nonterminal invocation-backed reserved/committed row contributes only a positive reservation-minus-persisted-cost residual. Tumbler tUSDC is excluded from production-USDC caps and reported in a separate block.