The Apple-Siri interoperability debate under the EU Digital Markets Act exposes a difficult technical question: how can third-party AI assistants gain meaningful access to device functions without forcing the platform to surrender privacy, security, or control over consequential actions? This paper proposes an execution-finality architecture in which an AI assistant may request an action, but the request itself has no power to make that action effective. Each consequential operation remains in a Non-Effective State until protected infrastructure validates the requester, resource, destination, user intent where required, freshness, revocation state, and policy conditions. Only then is narrowly scoped, non-bearer execution authority created. At the Finality Sink - the first boundary where the action can become externally effective - the system independently verifies that the real operation still matches what was authorized. Any mismatch, replay, substitution, expiry, or revocation causes fail-closed denial. The key principle is simple: Interoperability should grant participation, not uncontrolled execution authority. This offers a possible technical path through the DMA deadlock: third-party assistants could participate meaningfully without requiring broad reusable permissions, while platforms retain strong privacy, security, revocation, anti-replay, and final-effect controls. Execution-Finality Governance therefore reframes the problem from closed versus open to open participation with bounded, verifiable authority.