Fix Critical: streaming race, dirty buffer, dropped tail, fragmented header

- SeekBeyondBuffer and LoadTrackStreaming assign _activeStreamingTask before
  awaiting; DrainActiveStreamingTaskAsync awaits previous task before new
  stream starts, closing the concurrent-seek race on the JS StreamDecoder
- Always slice ArrayPool buffer to currentBytes before sending to JS interop;
  eliminates stale bytes from prior rentals reaching the audio decoder
- getSampleAlignedChunkSize accepts streamComplete flag; bypasses minimum
  chunk guard on final tail so trailing bytes are decoded, not dropped
- StreamDecoder accumulates headerSearchChunks until parseHeader succeeds,
  with 256 KB MAX_HEADER_SEARCH_BYTES bound; handles fragmented first chunks
  and extended WAV headers with LIST/INFO/JUNK chunks
- markStreamComplete early-returns when streamComplete already set to prevent
  double-drain and incorrect streamingCompleted flag after partial failure
- processedBytes advances only after successful decode; failed segments leave
  cursor in place rather than permanently skipping audio
- AudioInteropService.MarkStreamCompleteAsync wires C# loop exit to JS decoder
  ensuring tail drain fires even when Content-Length header is absent

DeepDrftWeb.Client/Services/AudioInteropService.cs

@@ -56,6 +56,11 @@ public class AudioInteropService : IAsyncDisposable
         return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.startStreamingPlayback", playerId);
     }
 
+    public async Task<AudioOperationResult> MarkStreamCompleteAsync(string playerId)
+    {
+        return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.markStreamComplete", playerId);
+    }
+
     public async Task<AudioOperationResult> EnsureAudioContextReady(string playerId)
     {
         return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.ensureAudioContextReady", playerId);