Use one immutable common-time event partition across Polymarket research tasks

[proerror77]

Parent\n\n#314\n\n## What to build\n\nCreate one shared EventCohortPartition artifact used by settlement, Up execution, and Down execution research. It deterministically records train event IDs, crossing-excluded event IDs, held-out event IDs, the common time boundary, and a digest. No evaluator or MCTS adapter may independently resplit rows.\n\nThe partition is upstream of ResearchSnapshot construction. It must be derived from #319's opaque independently verified ready-event/cohort projection and bind the ordered ready-entry/catalog identity. It must never include a ResearchSnapshot digest in its own hash; #320 binds the completed partition digest into the ResearchSnapshot.\n\n## Acceptance criteria\n\n- [ ] Partition input is authenticated ready-event metadata from #319 and output is a versioned immutable artifact with deterministic digest.\n- [ ] The partition digest binds the ordered ready-entry/catalog identity, market_id, reference-path window, common time boundary, and assignments; it does not bind a downstream ResearchSnapshot digest.\n- [ ] Every market_id appears in exactly one of train, crossing-excluded, or held-out.\n- [ ] Events whose reference path overlaps the time boundary are excluded rather than split by rows.\n- [ ] Settlement, Up, and Down tasks consume the exact same partition digest.\n- [ ] Held-out rows and labels are inaccessible to candidate generation, MCTS selection, and backpropagation.\n- [ ] Counterexamples cover row-random split, one market on both sides, crossing leakage, reordered input instability, task-specific resplitting, missing catalog members, and snapshot/partition hash cycles.\n\n## Blocked by\n\n#319\n#321\n\n## Out of scope\n\nMission schema beyond partition reference, evaluator metrics, MCTS search policy, snapshot construction, and runtime deployment.