test(api): guard the spread-only façade against interleave regrowth (R7.5 S3)

Add facade.test.ts asserting the two invariants that keep lib/api.ts safe:
no method name is defined in two slices (a silent last-wins spread override),
and the `api` object is exactly the union of the slices (nothing added inline
in the barrel, nothing lost). Verified to have teeth — an injected duplicate
key fails the collision test with a named message.

Closes R7.5: the api layer is now per-domain modules behind a mechanically
guarded spread-only façade.

frontend/src/lib/api/facade.test.ts

@@ -0,0 +1,85 @@
+import { describe, it, expect, beforeAll } from "vitest";
+
+// The R7.5 guard: lib/api.ts is a spread-only façade over per-domain slice modules. These tests
+// mechanise the two invariants that make that structure safe — so a future edit that reintroduces
+// the interleave (a method dropped into two slices → a silent last-wins spread override) or bloats
+// the barrel (a method defined inline instead of in a domain module) fails CI instead of shipping.
+
+type Slice = Record<string, unknown>;
+
+// Names line up with the modules below; kept as a list so the wiring stays a single source.
+const SLICE_MODULES = [
+  ["accountApi", () => import("./account")],
+  ["authApi", () => import("./auth")],
+  ["setupApi", () => import("./setup")],
+  ["feedApi", () => import("./feed")],
+  ["channelsApi", () => import("./channels")],
+  ["syncApi", () => import("./sync")],
+  ["playlistsApi", () => import("./playlists")],
+  ["tagsApi", () => import("./tags")],
+  ["notificationsApi", () => import("./notifications")],
+  ["savedViewsApi", () => import("./savedViews")],
+  ["quotaApi", () => import("./quota")],
+  ["adminApi", () => import("./admin")],
+  ["plexApi", () => import("./plex")],
+  ["messagingApi", () => import("./messaging")],
+  ["downloadsApi", () => import("./downloads")],
+] as const;
+
+let api: Slice;
+const slices: Record<string, Slice> = {};
+
+beforeAll(async () => {
+  // The runtime api graph pulls i18n at module-load, which reads localStorage/navigator. Under
+  // vitest's node env (no jsdom — see vitest.config.ts) stub the minimum so the import succeeds.
+  if (typeof globalThis.localStorage === "undefined") {
+    const store = new Map<string, string>();
+    globalThis.localStorage = {
+      getItem: (k: string) => store.get(k) ?? null,
+      setItem: (k: string, v: string) => void store.set(k, String(v)),
+      removeItem: (k: string) => void store.delete(k),
+      clear: () => store.clear(),
+      key: (i: number) => [...store.keys()][i] ?? null,
+      get length() {
+        return store.size;
+      },
+    } as Storage;
+  }
+  if (typeof globalThis.navigator === "undefined") {
+    globalThis.navigator = { language: "en" } as Navigator;
+  }
+  if (typeof globalThis.document === "undefined") {
+    globalThis.document = { documentElement: {} } as Document;
+  }
+
+  for (const [name, load] of SLICE_MODULES) {
+    const mod = (await load()) as Record<string, Slice>;
+    const slice = mod[name];
+    if (!slice) throw new Error(`slice module did not export ${name}`);
+    slices[name] = slice;
+  }
+  api = ((await import("../api")) as { api: Slice }).api;
+});
+
+describe("api façade is a collision-free spread of domain slices", () => {
+  it("defines no method name in more than one slice (object spread would silently drop one)", () => {
+    const owner = new Map<string, string>();
+    const collisions: string[] = [];
+    for (const [sliceName, slice] of Object.entries(slices)) {
+      for (const key of Object.keys(slice)) {
+        const prev = owner.get(key);
+        if (prev) collisions.push(`"${key}" is defined in both ${prev} and ${sliceName}`);
+        else owner.set(key, sliceName);
+      }
+    }
+    expect(collisions).toEqual([]);
+  });
+
+  it("exposes exactly the union of the slices — nothing added inline in the barrel, nothing lost", () => {
+    const union = new Set<string>();
+    for (const slice of Object.values(slices)) {
+      for (const key of Object.keys(slice)) union.add(key);
+    }
+    expect(new Set(Object.keys(api))).toEqual(union);
+  });
+});