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/test/store-registry.test.ts

@@ -196,7 +196,14 @@ test("resolveScopeGraph maps scopes to native store NURIs (no store-id leaks to
   expect(await resolveScopeGraph("private")).toBe("did:ng:PRIV");
   expect(await resolveScopeGraph("protected")).toBe("did:ng:PROT");
   expect(await resolveScopeGraph("public")).toBe("did:ng:PROT"); // co-located
-  expect(await resolveInboxAnchor()).toBe("did:ng:PRIV");
+  // The inbox anchor is now a DEDICATED inbox DOCUMENT (a reserved account's
+  // public scope doc, from docCreate) — NOT the private-store root — so inbox
+  // deposits don't bloat the shim graph. It is a real repo NURI and STABLE
+  // across calls (same reserved account → same document).
+  const anchor = await resolveInboxAnchor();
+  expect(anchor).toMatch(/^did:ng:o:doc/);
+  expect(anchor).not.toBe("did:ng:PRIV");
+  expect(await resolveInboxAnchor()).toBe(anchor); // stable
 });
 
 test("resolveScopeGraph falls back to the private store when no protected id is injected", async () => {