Implement isolated user-engine MVP

docs/development.md

@@ -10,9 +10,12 @@ tested immediately in local and agent environments.
 
 ```text
 src/user_engine/
+  adapters/    local standalone adapters and deterministic test doubles
   domain/      transport- and persistence-neutral domain schemas
+  errors.py    typed service exceptions for callers and future transports
   ports.py     adapter protocols for identity, authorization, events, audit,
                membership export, application bindings, and secrets
+  service.py   headless service API for the isolated MVP
   testing/     local fixtures for tests and examples
 tests/         standard-library unittest suite
 ```
@@ -40,3 +43,12 @@ Add new behavior in this order:
 2. local fixture or adapter;
 3. test that proves the boundary;
 4. infrastructure adapter or API surface.
+
+## Isolated MVP Surface
+
+The initial headless API is `UserEngineService`. It exposes health,
+readiness, `me`, user/account lifecycle, identity linking, application
+registration, catalog publication, profile writes, effective profile
+resolution, projections, audit inspection, and outbox inspection. The first
+store is `InMemoryUserEngineStore`, which carries an explicit schema version
+and migration hook so later database-backed stores have a contract to match.