feat(core-*): add Vitest configs + composition tests

core-api: appRouter exposes all 4 feature routers + blog procedure shape.
core-cms: payloadConfig registers all collections + globals.
core-trpc: client + provider exports verified.

Spec: §6.1, §6.6

packages/core-cms/src/payload.config.test.ts

@@ -0,0 +1,20 @@
+import { describe, it, expect } from "vitest";
+import config from "./payload.config";
+
+describe("payloadConfig composition", () => {
+  it("registers all feature collections", async () => {
+    const resolved = await config;
+    const slugs = resolved.collections?.map((c) => c.slug) ?? [];
+    expect(slugs).toEqual(
+      expect.arrayContaining(["users", "articles", "pages", "media"]),
+    );
+  });
+
+  it("registers all feature globals", async () => {
+    const resolved = await config;
+    const slugs = resolved.globals?.map((g) => g.slug) ?? [];
+    expect(slugs).toEqual(
+      expect.arrayContaining(["site-settings", "header"]),
+    );
+  });
+});