test: enforce per-directory coverage thresholds

entities + use-cases + controllers must hit 100% (95% branches).
Project-wide baseline remains 80/75/80/80. Tightening these directories
reflects the architectural intent: these are the pure-logic layers and
should be exhaustively tested.

Added @vitest/coverage-v8@^3 to core-typescript, auth, blog,
marketing-pages, and navigation devDependencies.

Excluded from coverage (legitimately untestable):
- src/di/bind-production.ts — InversifyJS startup bootstrap
- src/application/repositories/** — pure TypeScript interfaces
- src/application/services/** — pure TypeScript interfaces (auth)
- src/integrations/cms/** — declarative Payload CMS config
- src/entities/cookie.ts — pure type aliases (auth)
- src/ui/** — React Query helpers, integration-tested in apps

Tests added (6 new files / 1 extended):
- auth/src/entities/errors.test.ts — UnauthenticatedError, UnauthorizedError,
  AuthenticationError, InputParseError constructors
- blog/src/entities/errors.test.ts — ArticleNotFoundError (default + custom
  message), InputParseError
- marketing-pages/src/entities/errors.test.ts — PageNotFoundError, InputParseError
- marketing-pages/src/entities/site-settings.test.ts — siteSettingsSchema
  (valid, no description, empty name rejection)
- navigation/src/entities/header.test.ts — headerItemSchema and headerSchema
  validation paths
- navigation/src/.../payload-header.repository.test.ts — logo relation-object,
  scalar-id, null, and null-item-field branches for full branch coverage

Spec: §6.9

packages/navigation/src/entities/header.test.ts

@@ -0,0 +1,61 @@
+import { describe, expect, it } from "vitest";
+import { headerSchema, headerItemSchema } from "./header";
+
+describe("headerItemSchema", () => {
+  it("accepts a valid nav item", () => {
+    const result = headerItemSchema.parse({
+      label: "Home",
+      href: "/",
+    });
+    expect(result.label).toBe("Home");
+    expect(result.external).toBe(false);
+  });
+
+  it("accepts an external nav item", () => {
+    const result = headerItemSchema.parse({
+      label: "Blog",
+      href: "https://blog.example.com",
+      external: true,
+    });
+    expect(result.external).toBe(true);
+  });
+
+  it("rejects an empty label", () => {
+    expect(() => headerItemSchema.parse({ label: "", href: "/" })).toThrow();
+  });
+
+  it("rejects a label over 64 chars", () => {
+    expect(() =>
+      headerItemSchema.parse({ label: "x".repeat(65), href: "/" }),
+    ).toThrow();
+  });
+
+  it("rejects an empty href", () => {
+    expect(() =>
+      headerItemSchema.parse({ label: "Home", href: "" }),
+    ).toThrow();
+  });
+});
+
+describe("headerSchema", () => {
+  it("accepts a valid header with items", () => {
+    const result = headerSchema.parse({
+      items: [{ label: "Home", href: "/" }],
+    });
+    expect(result.items).toHaveLength(1);
+    expect(result.logoId).toBeUndefined();
+  });
+
+  it("accepts a header with a logoId", () => {
+    const result = headerSchema.parse({
+      logoId: "logo-abc",
+      items: [],
+    });
+    expect(result.logoId).toBe("logo-abc");
+  });
+
+  it("accepts a header with no items", () => {
+    const result = headerSchema.parse({ items: [] });
+    expect(result.items).toHaveLength(0);
+  });
+});