feat: add durable store conformance harness

docs/contracts.md

@@ -231,6 +231,21 @@ adapter or provider concerns outside the domain service.
 
 ## Migration Contract
 
-The isolated store exposes `SCHEMA_VERSION = 0001_initial` and a `migrate`
-hook. Database-backed stores must expose equivalent readiness semantics before
-they are accepted by platform adapters.
+`user_engine.migrations` exposes the ordered durable-store manifest,
+`LATEST_SCHEMA_VERSION`, logical record types, and adapter-neutral diagnostic
+count keys. The isolated store's `SCHEMA_VERSION` is derived from that manifest
+and its `migrate` hook must be idempotent. Database-backed stores must expose
+equivalent readiness semantics before they are accepted by platform adapters.
+
+Provider-backed Postgres adapters can use
+`migrations/postgres/0001_user_engine_store.sql` as the bootstrap contract or
+translate it into their own migration framework while preserving schema-version
+tracking, logical record uniqueness, audit durability, and pending-outbox
+reads.
+
+Future adapters should run
+`user_engine.testing.assert_user_engine_store_conformance(testcase, factory)`
+with a factory that returns a fresh store. The harness covers readiness,
+idempotent migration, core save/read/query behavior, transaction rollback,
+outbox ordering, and diagnostics that expose counts without raw factor or
+profile values.