feat(spotify): report resolved backend + binaryAvailable on /status; share backend resolver [S4.2]

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

src/music/spotify/backend-select.test.ts

@@ -0,0 +1,12 @@
+import { describe, it, expect } from "vitest";
+import { resolveSpotifyBackendKind as pick } from "./backend-select.js";
+describe("resolveSpotifyBackendKind", () => {
+  it("auto: go present -> go-librespot", () => expect(pick("auto", true, true)).toBe("go-librespot"));
+  it("auto: go absent, rust present -> librespot", () => expect(pick("auto", false, true)).toBe("librespot"));
+  it("auto: neither -> null", () => expect(pick("auto", false, false)).toBeNull());
+  it("go-librespot: present -> go-librespot", () => expect(pick("go-librespot", true, true)).toBe("go-librespot"));
+  it("go-librespot: absent -> null even if rust present", () => expect(pick("go-librespot", false, true)).toBeNull());
+  it("librespot: present -> librespot", () => expect(pick("librespot", true, true)).toBe("librespot"));
+  it("librespot: absent -> null even if go present", () => expect(pick("librespot", true, false)).toBeNull());
+  it("auto default fallthrough matches auto", () => expect(pick("auto", true, false)).toBe("go-librespot"));
+});

src/music/spotify/backend-select.ts

@@ -0,0 +1,25 @@
+/** Which concrete backend runs for a given config + host binary availability. */
+export type SpotifyBackendKind = "go-librespot" | "librespot";
+
+/**
+ * Pure backend selection shared by SpotifyController.chooseBackend() (per-bot)
+ * and the web /status endpoint (process-wide). Booleans in, no IO — the caller
+ * supplies platform+binary presence.
+ */
+export function resolveSpotifyBackendKind(
+  backend: "auto" | "go-librespot" | "librespot",
+  goPresent: boolean,
+  rustPresent: boolean,
+): SpotifyBackendKind | null {
+  switch (backend) {
+    case "go-librespot":
+      return goPresent ? "go-librespot" : null;
+    case "librespot":
+      return rustPresent ? "librespot" : null;
+    case "auto":
+    default:
+      if (goPresent) return "go-librespot";
+      if (rustPresent) return "librespot";
+      return null;
+  }
+}

src/music/spotify/controller.ts

@@ -29,9 +29,11 @@ import {
   type OAuthTokenStore,
 } from "./spotify-oauth.js";
 import { SpotifyConnectApi } from "./connect-api.js";
-
-/** Which concrete backend the controller will run for this host + config. */
-export type SpotifyBackendKind = "go-librespot" | "librespot";
+import {
+  resolveSpotifyBackendKind,
+  type SpotifyBackendKind,
+} from "./backend-select.js";
+export type { SpotifyBackendKind }; // keep the name exported for existing importers
 
 /**
  * Minimal file-backed OAuth token store used when the caller does not inject a
@@ -156,17 +158,11 @@ export class SpotifyController extends EventEmitter {
    *                     librespot present, else null.
    */
   chooseBackend(): SpotifyBackendKind | null {
-    switch (this.config.backend) {
-      case "go-librespot":
-        return this.goPresent() ? "go-librespot" : null;
-      case "librespot":
-        return this.rustPresent() ? "librespot" : null;
-      case "auto":
-      default:
-        if (this.goPresent()) return "go-librespot";
-        if (this.rustPresent()) return "librespot";
-        return null;
-    }
+    return resolveSpotifyBackendKind(
+      this.config.backend,
+      this.goPresent(),
+      this.rustPresent(),
+    );
   }
 
   /** enabled in config AND a backend is selectable (platform + binary present). */