feat(spotify): add go-librespot REST client + WS event client (Stage 2)

GoLibrespotRestClient wraps axios (injectable via deps.http) for
/player/play|pause|resume|stop|seek, GET /status, GET / ping; ping/getStatus
swallow errors to false/null, mutating ops reject. GoLibrespotEventClient
(EventEmitter) parses {type,data} /events frames and re-emits type with data,
reconnects on close with capped backoff, stop() tears down. TDD with a mock
AxiosInstance and a fake WebSocket (no real binary/network).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/music/spotify/go-librespot-api.ts

@@ -0,0 +1,187 @@
+import { EventEmitter } from "node:events";
+import axios, { type AxiosInstance } from "axios";
+import WebSocket from "ws";
+
+export interface GoLibrespotStatusTrack {
+  uri: string;
+  name: string;
+  artist_names: string[];
+  album_name: string;
+  album_cover_url: string | null;
+  position: number;
+  duration: number;
+}
+
+export interface GoLibrespotStatus {
+  stopped: boolean;
+  paused: boolean;
+  buffering: boolean;
+  track: GoLibrespotStatusTrack | null;
+}
+
+export class GoLibrespotRestClient {
+  private http: AxiosInstance;
+
+  constructor(baseUrl: string, deps?: { http?: AxiosInstance }) {
+    this.http =
+      deps?.http ??
+      axios.create({
+        baseURL: baseUrl,
+        timeout: 10000,
+        headers: { "Content-Type": "application/json" },
+      });
+  }
+
+  async ping(): Promise<boolean> {
+    try {
+      const res = await this.http.get("/");
+      return res.status === 200;
+    } catch {
+      return false;
+    }
+  }
+
+  async playTrack(uri: string): Promise<void> {
+    await this.http.post("/player/play", { uri });
+  }
+
+  async pause(): Promise<void> {
+    await this.http.post("/player/pause");
+  }
+
+  async resume(): Promise<void> {
+    await this.http.post("/player/resume");
+  }
+
+  async stop(): Promise<void> {
+    await this.http.post("/player/stop");
+  }
+
+  async seek(ms: number): Promise<void> {
+    await this.http.post("/player/seek", { position: ms, relative: false });
+  }
+
+  async getStatus(): Promise<GoLibrespotStatus | null> {
+    try {
+      const res = await this.http.get("/status");
+      const d = res.data ?? {};
+      const t = d.track;
+      return {
+        stopped: Boolean(d.stopped),
+        paused: Boolean(d.paused),
+        buffering: Boolean(d.buffering),
+        track: t
+          ? {
+              uri: t.uri ?? "",
+              name: t.name ?? "",
+              artist_names: Array.isArray(t.artist_names) ? t.artist_names : [],
+              album_name: t.album_name ?? "",
+              album_cover_url: t.album_cover_url ?? null,
+              position: t.position ?? 0,
+              duration: t.duration ?? 0,
+            }
+          : null,
+      };
+    } catch {
+      return null;
+    }
+  }
+}
+
+export type GoLibrespotEventType =
+  | "metadata"
+  | "playing"
+  | "paused"
+  | "not_playing"
+  | "stopped"
+  | "will_play"
+  | "seek"
+  | "active"
+  | "inactive"
+  | "volume"
+  | "playback_ready";
+
+interface WsLike {
+  on(event: string, cb: (...args: any[]) => void): void;
+  close(): void;
+}
+type WebSocketCtor = new (url: string) => WsLike;
+
+const INITIAL_RECONNECT_MS = 500;
+const MAX_RECONNECT_MS = 10000;
+
+export class GoLibrespotEventClient extends EventEmitter {
+  private wsUrl: string;
+  private WebSocketCtor: WebSocketCtor;
+  private ws: WsLike | null = null;
+  private stopped = false;
+  private reconnectDelay = INITIAL_RECONNECT_MS;
+  private reconnectTimer: ReturnType<typeof setTimeout> | null = null;
+
+  constructor(wsUrl: string, deps?: { WebSocketCtor?: WebSocketCtor }) {
+    super();
+    this.wsUrl = wsUrl;
+    this.WebSocketCtor = deps?.WebSocketCtor ?? (WebSocket as unknown as WebSocketCtor);
+  }
+
+  start(): void {
+    this.stopped = false;
+    this.connect();
+  }
+
+  stop(): void {
+    this.stopped = true;
+    if (this.reconnectTimer) {
+      clearTimeout(this.reconnectTimer);
+      this.reconnectTimer = null;
+    }
+    if (this.ws) {
+      this.ws.close();
+      this.ws = null;
+    }
+  }
+
+  private connect(): void {
+    if (this.stopped) return;
+    const ws = new this.WebSocketCtor(this.wsUrl);
+    this.ws = ws;
+    ws.on("open", () => {
+      this.reconnectDelay = INITIAL_RECONNECT_MS;
+    });
+    ws.on("message", (buf: unknown) => this.handleMessage(buf));
+    ws.on("close", () => {
+      this.ws = null;
+      this.scheduleReconnect();
+    });
+    ws.on("error", (err: unknown) => {
+      if (this.listenerCount("error") > 0) this.emit("error", err);
+    });
+  }
+
+  private handleMessage(buf: unknown): void {
+    let parsed: unknown;
+    try {
+      const text = Buffer.isBuffer(buf)
+        ? buf.toString("utf8")
+        : typeof buf === "string"
+          ? buf
+          : String(buf);
+      parsed = JSON.parse(text);
+    } catch {
+      return;
+    }
+    if (parsed && typeof (parsed as any).type === "string") {
+      this.emit((parsed as any).type, (parsed as any).data ?? {});
+    }
+  }
+
+  private scheduleReconnect(): void {
+    if (this.stopped || this.reconnectTimer) return;
+    const delay = this.reconnectDelay;
+    this.reconnectDelay = Math.min(delay * 2, MAX_RECONNECT_MS);
+    this.reconnectTimer = setTimeout(() => {
+      this.reconnectTimer = null;
+      this.connect();
+    }, delay);
+  }
+}