feat: qbz-1 streaming, gapless prefetch, accurate scrobbling, Range-seek

Port proven playback architecture from qbqt fork:
- Bounded VecDeque buffer with condvar backpressure (4MB cap)
- decrypt_and_extract_frames for clean FLAC frame extraction from ISOBMFF
- Cancel+restart seeking with sub-segment sample skipping
- start_prefetch / QueueNext for gapless transitions with pre-started downloads
- track_transitioned signaling for scrobbler during gapless playback
- Range-request HTTP seeking for non-segmented (MP3) tracks
- OnceLock HTTP client singleton with cancel-aware chunked downloads
- Accumulated listening time scrobbling (prevents false scrobbles from seeking)
- Array-format Last.fm scrobble params (artist[0], track[0], etc.)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust/src/player/output.rs

@@ -14,7 +14,6 @@ const RING_BUFFER_SIZE: usize = 32 * 1024;
 
 pub struct AudioOutput {
     ring_buf_producer: rb::Producer<f32>,
-    viz_producer: Option<rb::Producer<f32>>,
     _stream: cpal::Stream,
     pub sample_rate: u32,
     pub channels: usize,
@@ -52,17 +51,12 @@ impl AudioOutput {
 
         Ok(Self {
             ring_buf_producer: producer,
-            viz_producer: None,
             _stream: stream,
             sample_rate,
             channels,
         })
     }
 
-    pub fn set_viz_producer(&mut self, producer: rb::Producer<f32>) {
-        self.viz_producer = Some(producer);
-    }
-
     pub fn write(
         &mut self,
         decoded: AudioBufferRef<'_>,
@@ -75,13 +69,13 @@ impl AudioOutput {
         );
         sample_buf.copy_interleaved_ref(decoded);
         let samples: Vec<f32> = sample_buf.samples().iter().map(|s| s * volume).collect();
+        self.write_samples(&samples, stop)
+    }
 
-        // Best-effort copy for visualizer (non-blocking, ok to drop samples)
-        if let Some(ref mut viz) = self.viz_producer {
-            let _ = viz.write(&samples);
-        }
-
-        let mut remaining = &samples[..];
+    /// Write pre-converted interleaved f32 samples directly to the ring buffer.
+    /// Returns early (without error) if `stop` is set.
+    pub fn write_samples(&mut self, samples: &[f32], stop: &Arc<AtomicBool>) -> Result<()> {
+        let mut remaining = samples;
         while !remaining.is_empty() {
             if stop.load(Ordering::SeqCst) {
                 return Ok(());