fix(client): per-entity write round-trip + dedicated inbox anchor

Real-broker validation of the per-document model surfaced round-trip breaks the
fake-ng unit tests missed:
- The inbox anchor was the shim graph itself, making loadShim ~60s; give the
  inbox its own document so anchor resolution is fast and isolated.
- store-registry adjustments so per-entity documents created via the SDK are
  indexed and readable back through the scope fan-out.
docs/simulation.md updated. 89 tests pass; tsc rc=0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/client/src/store-registry.ts

@@ -315,15 +315,31 @@ export async function resolveScopeGraph(scope: Scope): Promise<Nuri> {
   return scopeStoreNuri(scope);
 }
 
+/**
+ * The reserved account that OWNS the shared registration-inbox document. Like the
+ * discovery index's special account, it lives in the reserved namespace (no user
+ * can produce this key) and only HOSTS a document — its `public` scope document is
+ * the inbox anchor. Disappears at migration (native per-document inboxes).
+ */
+const INBOX_ANCHOR_ACCOUNT = reservedAccount("inbox");
+
 /**
  * The inbox anchor NURI for the current session (where emulated inbox deposits
- * physically land). SDK-shaped: the consumer never resolves the private store
- * itself. In the polyfill this is the shared wallet's private store (a real
- * repo NURI — required because the broker rejects a `urn:` anchor); at
- * migration it becomes the host's native inbox and this resolution moves here.
+ * physically land). SDK-shaped: the consumer never resolves a store itself.
+ *
+ * This is a DEDICATED inbox DOCUMENT (a reserved account's public scope document —
+ * a real repo NURI from `docCreate`, stable across clients via the shim), NOT the
+ * shared wallet's private-store root. Reason (perf + hygiene): the shim (the
+ * account→document trust root) lives in the private-store graph and is scanned on
+ * every `loadShim`; routing every inbox deposit into that SAME graph bloats it
+ * without bound (thousands of deposit triples across sessions), turning `loadShim`
+ * into a multi-second full-graph scan. A separate inbox document keeps the shim
+ * graph small and the deposits isolated. At migration this becomes the host's
+ * native per-document inbox and the resolution moves here.
  */
 export async function resolveInboxAnchor(): Promise<Nuri> {
-  return scopeStoreNuri("private");
+  const record = await ensureAccount(INBOX_ANCHOR_ACCOUNT);
+  return record.docPublic;
 }
 
 // --- per-entity documents + per-scope index -------------------------------