Expand description
Stage application — runtime-side apply_stage.
Lives in runtime/ so the layer DAG abi → state → runtime → persist
is preserved (R4-X) — placing it on Instance would force state to
import runtime::StepStage.
apply_stage borrows the stage &mut and drains the buckets it commits,
so the kernel can reuse one StepStage scratch across actions. The
rollback path needs no separate function: the kernel simply skips
apply_stage and clear()s the scratch before the next action.
Application is panic-free (totality contract): every operation
uses saturating arithmetic and if let Some(...) style guards.