Why: Material runs could spend the remaining budget on checkpoints and leave no final response.
Evidence: go test ./... -count=1; go test -race ./internal/runtime -run TestMaterialFinalReserve -count=1; offline replay of the prior A01 usage boundary.
Impact: Material subagents enter final-only mode or stop before another provider request; ordinary subagents retain their existing behavior.
Refs: U3 quality revision r0.2, Task 4