feat(features): contract suites for all repository interfaces

Each repository interface now has a contract suite under
src/__contracts__/. Both Mock and Payload implementations run the
same suite, eliminating mock-vs-real drift. Payload impls back the
contract with an in-memory stub via vi.mock('payload') + a small
buildPayloadStub helper.

Spec: §5.2, §6.4

packages/navigation/src/__contracts__/header-repository.contract.ts

@@ -0,0 +1,47 @@
+import { it, expect, beforeEach } from "vitest";
+import { defineContractSuite } from "@repo/core-testing/contract";
+import type { IHeaderRepository } from "../application/repositories/header-repository.interface.js";
+
+/**
+ * Contract for IHeaderRepository.
+ *
+ * Header is a singleton (Payload Global). The interface exposes only
+ * getHeader(). The contract verifies the shape of the return value.
+ */
+export const headerRepositoryContract =
+  defineContractSuite<IHeaderRepository>(
+    "IHeaderRepository",
+    ({ buildSubject }) => {
+      let repo: IHeaderRepository;
+
+      beforeEach(async () => {
+        repo = await buildSubject();
+      });
+
+      // --- getHeader ---
+
+      it("getHeader returns an object with an items array", async () => {
+        const header = await repo.getHeader();
+        expect(header).toBeDefined();
+        expect(header.items).toBeInstanceOf(Array);
+      });
+
+      it("getHeader items have label, href, and external fields", async () => {
+        const header = await repo.getHeader();
+        for (const item of header.items) {
+          expect(typeof item.label).toBe("string");
+          expect(item.label.length).toBeGreaterThan(0);
+          expect(typeof item.href).toBe("string");
+          expect(item.href.length).toBeGreaterThan(0);
+          expect(typeof item.external).toBe("boolean");
+        }
+      });
+
+      it("getHeader logoId is string or undefined", async () => {
+        const header = await repo.getHeader();
+        expect(
+          header.logoId === undefined || typeof header.logoId === "string",
+        ).toBe(true);
+      });
+    },
+  );