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Continues the Phase 1 contract doc with the same wire-contract-only style. All three build on Sprint 0.1's answered decisions - no further business input needed to start implementation once backend ownership is confirmed: - Phase 2: canonical Order/OrderLine/Fulfillment/OrderEvent per the unified multi-seller decision (one Order, per-seller Fulfillment groups), event bus, Notification Center contract. - Phase 3: Product/Offer split, InventoryRecord, publish-time executability validation (the mechanism behind "no branch may distinguish an inspector from a normal buyer"). - Phase 4: generic config-driven connector framework per the "no fixed marketplace list" decision - onboarding a new partner is configuration against a fixed pipeline, not a bespoke integration. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>