ADR 0001: Modular monolith and provider contract

Status: Accepted

UnifyEMPI uses a modular monolith with separate API and MLLP processes. Domain and application modules remain version- and provider-neutral. IIdentityRegistryStore is the only persistence boundary.

This keeps identity decisions transactional and explainable while allowing the two network protocols to scale independently. It avoids distributed transaction failure modes in the safety-critical link path. A deployment selects exactly one provider; runtime assembly hot-swapping is intentionally excluded.

Provider implementations must supply atomic mutations, optimistic concurrency, idempotency, opaque pagination, and bounded candidate lookup and must pass the common contract suite.