feat(spotify): add Rust librespot binary resolver (Stage 3 Task 1)

Append isRustLibrespotSupported/pickLibrespotPath/findLibrespot/
checkLibrespotAvailable/resetLibrespotBinaryCache to binary.ts, mirroring
the go-librespot resolver. Supported on all platforms (pipe->stdout),
resolves librespot.exe on win32, caches only positive --version probes.

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

src/music/spotify/binary.ts

@@ -91,3 +91,102 @@ export function resetGoLibrespotBinaryCache(): void {
   cachedAvailable = false;
   pendingCheck = null;
 }
+
+// ---------------------------------------------------------------------------
+// Rust librespot (librespot-org) resolver — mirrors the go-librespot fns
+// above. Unlike go-librespot, Rust librespot's `--backend pipe` writes PCM to
+// *stdout* on every platform (no FIFO, no audio device), so it is supported
+// on Windows/macOS/Linux alike and the binary is named librespot.exe on win32.
+// ---------------------------------------------------------------------------
+
+/**
+ * True on ALL platforms. The Rust librespot pipe backend writes raw bytes to
+ * process stdout, which Node's spawned child.stdout receives unmodified on
+ * Windows too — so there is no platform gate here (contrast
+ * isGoLibrespotSupported, which is Linux-only).
+ */
+export function isRustLibrespotSupported(): boolean {
+  return true;
+}
+
+/**
+ * Pure resolver core behind findLibrespot(). Returns the first candidate that
+ * is either a bare command name (left for execFile to resolve via PATH) or an
+ * existing bin/ file. Exported so tests can inject candidates + a fake
+ * existence predicate and need no real binary on disk. Mirrors
+ * pickGoLibrespotPath but keys off the win32 exe name.
+ */
+export function pickLibrespotPath(
+  candidates: string[],
+  exists: (p: string) => boolean,
+): string {
+  const exe = process.platform === "win32" ? "librespot.exe" : "librespot";
+  for (const c of candidates) {
+    // bin/ paths only count when the file is actually present; bare names are
+    // returned unconditionally and resolved later via PATH.
+    const isBinPath = c.includes(join("bin", "librespot"));
+    if (!isBinPath || exists(c)) return c;
+  }
+  return exe;
+}
+
+/**
+ * Resolve the Rust librespot binary path: project bin/ dir first, then PATH.
+ * On win32 both the bin/librespot.exe candidate and the bare "librespot.exe"
+ * fallback are used so a PATH-installed librespot.exe (scoop/choco) resolves.
+ */
+export function findLibrespot(): string {
+  const exe = process.platform === "win32" ? "librespot.exe" : "librespot";
+  // src/music/spotify -> ../../../bin (same depth as findGoLibrespot).
+  const binExe = join(__dirname, "..", "..", "..", "bin", exe);
+  const binBare = join(__dirname, "..", "..", "..", "bin", "librespot");
+  return pickLibrespotPath([binExe, binBare, exe], existsSync);
+}
+
+// Injectable `--version` probe. Defaults to the real execFile call; tests
+// override it so checkLibrespotAvailable() needs no real binary. Keeps the
+// public checkLibrespotAvailable() signature param-free per the contract.
+type LibrespotVersionProbe = (bin: string) => Promise<void>;
+const realLibrespotProbe: LibrespotVersionProbe = async (bin) => {
+  await execFileAsync(bin, ["--version"], { timeout: 5_000, maxBuffer: 1024 });
+};
+let librespotVersionProbe: LibrespotVersionProbe = realLibrespotProbe;
+
+/** Test hook: override the `--version` probe, or restore the default with null. */
+export function __setLibrespotVersionProbe(
+  probe: LibrespotVersionProbe | null,
+): void {
+  librespotVersionProbe = probe ?? realLibrespotProbe;
+}
+
+/**
+ * Availability check for Rust librespot. No platform gate (supported
+ * everywhere). Runs `librespot --version` (5s timeout) and caches ONLY the
+ * positive result — a missing binary is retried on the next call so the
+ * operator can install it (cargo/scoop/choco) without restarting the server.
+ */
+let rustCachedAvailable = false;
+let rustPendingCheck: Promise<boolean> | null = null;
+export async function checkLibrespotAvailable(): Promise<boolean> {
+  if (!isRustLibrespotSupported()) return false;
+  if (rustCachedAvailable) return true;
+  if (rustPendingCheck) return rustPendingCheck;
+  rustPendingCheck = (async () => {
+    try {
+      await librespotVersionProbe(findLibrespot());
+      rustCachedAvailable = true;
+      return true;
+    } catch {
+      return false;
+    } finally {
+      rustPendingCheck = null;
+    }
+  })();
+  return rustPendingCheck;
+}
+
+/** Force re-detection on the next call (for tests). */
+export function resetLibrespotBinaryCache(): void {
+  rustCachedAvailable = false;
+  rustPendingCheck = null;
+}