Add fixture-only execution materialization and refresh (#1500)

packages/mosaic/src/fixture-store.mjs

@@ -0,0 +1,31 @@
+// In-memory synthetic store only; no filesystem or production backend.
+import { refuse, validateFixtureCredential } from './execution.mjs';
+const stores = new WeakSet();
+export const isFixtureStore = store => stores.has(store);
+export function createFixtureCredentialStore(initial) {
+  const records = new Map(), locks = new Set();
+  for (const [ref, value] of Object.entries(initial)) {
+    if (!/^[a-z0-9][a-z0-9._-]{0,63}\/[a-z0-9][a-z0-9._-]{0,63}$/.test(ref)) refuse('invalid-account-ref');
+    records.set(ref, validateFixtureCredential(value, value?.type ?? 'none'));
+  }
+  const store = Object.freeze({
+    fixtureOnly: true,
+    async transaction(refs, callback) {
+      const keys = [...new Set(refs)].sort();
+      for (const key of keys) {
+        if (!records.has(key)) refuse('missing-fixture-credential');
+        if (locks.has(key)) refuse('credential-busy');
+      }
+      keys.forEach(key => locks.add(key));
+      try {
+        const draft = new Map(keys.map(key => [key, structuredClone(records.get(key))]));
+        const result = await callback(draft);
+        for (const key of keys) validateFixtureCredential(draft.get(key), records.get(key)?.type ?? 'none');
+        keys.forEach(key => records.set(key, structuredClone(draft.get(key))));
+        return result;
+      } finally { keys.forEach(key => locks.delete(key)); }
+    },
+  });
+  stores.add(store);
+  return store;
+}