fix(player): start frame loop only after ffmpeg spawns in jdymusic path

PR #54's playViaPowerShellDownload called startFrameLoop() right after
spawning the PowerShell downloader, before ffmpeg existed. The loop's
"no ffmpeg + empty buffer → emit trackEnd" branch fired on the very
first tick (~25ms), skipping every jdymusic song. Visible as: each
PowerShell download sessionId logged "Track ended, advancing queue"
before the download completed, so the queue burned through every
track in a few seconds.

Move startFrameLoop() into spawnFfmpegFromFile() where ffmpeg is
known to be alive and producing PCM. state = "playing" still flips
in playViaPowerShellDownload so external observers see the right
status during download.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/audio/player.ts

@@ -279,8 +279,12 @@ export class AudioPlayer extends EventEmitter {
       this.emit("error", err);
     });
 
+    // Mark playing but DO NOT start the frame loop here — the loop's
+    // "no ffmpeg + empty buffer → trackEnd" branch would fire on the very
+    // first tick, before the PowerShell download even completes. The
+    // frame loop is started inside spawnFfmpegFromFile() once ffmpeg is
+    // alive and producing PCM.
     this.state = "playing";
-    this.startFrameLoop();
   }
 
   private spawnFfmpegFromFile(tempFile: string, seekSeconds: number, sessionId: number): void {
@@ -329,6 +333,9 @@ export class AudioPlayer extends EventEmitter {
         this.emit("error", err);
       }
     });
+
+    // Now that ffmpeg is producing PCM, run the frame loop.
+    this.startFrameLoop();
   }
 
   stop(): void {