refactor(plugins): make runtime registry lazy

src/plugins/runtime.test.ts

@@ -1,6 +1,7 @@
 import { afterEach, describe, expect, it } from "vitest";
 import { createEmptyPluginRegistry } from "./registry.js";
 import {
+  getActivePluginRegistry,
   pinActivePluginHttpRouteRegistry,
   releasePinnedPluginHttpRouteRegistry,
   resetPluginRuntimeStateForTest,
@@ -14,6 +15,12 @@ describe("plugin runtime route registry", () => {
     resetPluginRuntimeStateForTest();
   });
 
+  it("stays empty until a caller explicitly installs or requires a registry", () => {
+    resetPluginRuntimeStateForTest();
+
+    expect(getActivePluginRegistry()).toBeNull();
+  });
+
   it("keeps the pinned route registry when the active plugin registry changes", () => {
     const startupRegistry = createEmptyPluginRegistry();
     const laterRegistry = createEmptyPluginRegistry();