Three-hard-boundary model

This subject has provenance in frozen ADR history (0016-three-hard-boundary-model.md) but no living decision has been ratified yet under the topic-keyed decision registry (context/contracts/decision-registry-contract.md). This stub records that the subject is open and links the frozen ADR(s) as provenance; it is not a decision.

When a living decision is ratified for Three-hard-boundary model, update this file’s status to current, add rationale, and keep the adr evidence links as provenance for the history that led here.