[spec 9] Incremental log state in the node engine #5165

(AI) Follow-up from #4661 (spec 9, the experimental single-orchestrator runtime, opt-in via WORKFLOW_SPEC_VERSION=9). Tracked as follow-up 6 on the spec 9 changelog.

Each in-band write rescans the whole log (maxEventSlot, appendUniqueEvents, a full sort in mergeReportedEvents, findEventSlotGap), so CPU per step grows with log length, about O(n) per write and O(n²) per delivery. The duplicate slotSnapshot() is gone (b489e7561b). Before default-on: a running max slot, an id Set owned by the log and a dense-prefix watermark, reset on full load and repair, plus a ms-per-step benchmark at 250 against 2000 steps. Spec 8 has the same linear shape at a smaller constant. 4365355561.