Client class red: literate usage tests and stub

test/client/usage.test.ts is a tutorial-as-test-suite that walks
through the entire quack API in order: login, whoami, listCollections,
listFiles, downloadFile, downloadThumbnail, toJSON/fromJSON, logout.

Each it() block is a self-contained example with prose commentary
explaining what the code does and why, with code samples showing the
API as a consumer would use it. The mock server performs real SRP and
crypto so the test data is structurally identical to production.

8 tests, all failing against the stub.

src/client.ts

@@ -0,0 +1,72 @@
+// Stub: see the README "Development workflow" section for TDD policy.
+
+import type { ApiClientOptions } from "./api/client.js";
+import type { Collection, EnteFile } from "./model/types.js";
+import type { DownloadResult } from "./download/index.js";
+
+export interface LoginOptions {
+    email: string;
+    password: string;
+    totp?: () => Promise<string>;
+    emailOTP?: () => Promise<string>;
+    apiOptions?: ApiClientOptions;
+}
+
+export interface ClientSnapshot {
+    email: string;
+    userID: number;
+    token: string;
+    masterKey: string;
+    secretKey: string;
+    publicKey: string;
+}
+
+export class Client {
+    static async login(_opts: LoginOptions): Promise<Client> {
+        throw new Error("Client.login not implemented");
+    }
+
+    static fromJSON(
+        _snapshot: ClientSnapshot,
+        _apiOptions?: ApiClientOptions,
+    ): Client {
+        throw new Error("Client.fromJSON not implemented");
+    }
+
+    whoami(): { email: string; userID: number } {
+        throw new Error("Client.whoami not implemented");
+    }
+
+    toJSON(): ClientSnapshot {
+        throw new Error("Client.toJSON not implemented");
+    }
+
+    logout(): void {
+        throw new Error("Client.logout not implemented");
+    }
+
+    async listCollections(): Promise<Collection[]> {
+        throw new Error("Client.listCollections not implemented");
+    }
+
+    async listFiles(
+        _collectionID: number,
+        _collectionKey: Uint8Array,
+    ): Promise<EnteFile[]> {
+        throw new Error("Client.listFiles not implemented");
+    }
+
+    async downloadFile(
+        _file: EnteFile,
+        _outPath?: string,
+    ): Promise<DownloadResult> {
+        throw new Error("Client.downloadFile not implemented");
+    }
+
+    async downloadThumbnail(
+        _file: EnteFile,
+        _outPath?: string,
+    ): Promise<DownloadResult> {
+        throw new Error("Client.downloadThumbnail not implemented");
+    }
+}