Add [BP-DIAG] back-pressure instrumentation for Phase 21.4 browser run

Temporary, grep-tagged diagnostics at the read-loop pause, the scheduler
latch, and the chunk-result path to show whether ProductionPaused latches,
reaches C#, and parks the loop. Strip once the cause is confirmed.

DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs

@@ -35,6 +35,14 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
     public int BufferedChunks { get; private set; } = 0;
     public bool IsSeekingBeyondBuffer { get; private set; } = false;
 
+    // ───────────────────────────────────────────────────────────────────────────────────────────
+    // [BP-DIAG] Phase 21.4 back-pressure diagnostic. TEMPORARY — strip once the cause is confirmed
+    // in Daniel's browser run. Logs every Nth chunk's ProductionPaused flag plus pause-poll
+    // enter/exit so a grep for "[BP-DIAG]" in the WASM console tells whether the read loop ever sees
+    // the pause signal and whether the poll actually holds. Throttled by chunk count to avoid flooding.
+    private const int BpDiagChunkLogEvery = 16;
+    // ───────────────────────────────────────────────────────────────────────────────────────────
+
     private bool _streamingPlaybackStarted = false;
     private CancellationTokenSource? _streamingCancellation;
     private Task? _activeStreamingTask;
@@ -386,6 +394,7 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
             buffer = ArrayPool<byte>.Shared.Rent(MaxBufferSize); // Rent larger buffer to accommodate adaptive sizing
             int currentBytes;
             var readTimer = System.Diagnostics.Stopwatch.StartNew();
+            var bpDiagChunkIndex = 0; // [BP-DIAG] per-stream chunk counter for throttled logging
             
             do
             {
@@ -420,6 +429,20 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
                     HeaderParsed = chunkResult.HeaderParsed;
                     BufferedChunks = chunkResult.BufferCount;
 
+                    // [BP-DIAG] Phase 21.4 — throttled per-chunk view of the back-pressure signal as
+                    // the C# loop sees it. If ProductionPaused never logs True while bytes keep
+                    // flowing, the break is upstream (JS latch / lookahead math); if it logs True but
+                    // the transfer still races to 100%, the break is the transport (browser buffered
+                    // the whole body, SetBrowserResponseStreamingEnabled not in effect). TEMPORARY.
+                    if (bpDiagChunkIndex % BpDiagChunkLogEvery == 0)
+                    {
+                        _logger.LogInformation(
+                            "[BP-DIAG] chunk #{Chunk} bytesRead={Bytes} totalRead={Total} bufferCount={BufCount} canStart={CanStart} productionPaused={Paused} isPaused={IsPaused}",
+                            bpDiagChunkIndex, currentBytes, totalBytesRead, chunkResult.BufferCount,
+                            chunkResult.CanStartStreaming, chunkResult.ProductionPaused, IsPaused);
+                    }
+                    bpDiagChunkIndex++;
+
                     // Set duration from WAV header when available (only set once)
                     if (chunkResult.Duration.HasValue && Duration == null)
                     {
@@ -487,6 +510,15 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
                     // no separate cancellation path, no stale read racing a reinit.
                     if (chunkResult.ProductionPaused)
                     {
+                        // [BP-DIAG] Phase 21.4 — the read loop is ENTERING the pause-poll: reads have
+                        // stopped, the socket should now stall and the transfer should plateau. If you
+                        // see this line but the network transfer still completes, the transport is
+                        // buffered (streaming flag not in effect). TEMPORARY.
+                        _logger.LogInformation(
+                            "[BP-DIAG] ENTER pause-poll at chunk #{Chunk} totalRead={Total} isPaused={IsPaused}",
+                            bpDiagChunkIndex, totalBytesRead, IsPaused);
+                        var bpDiagPollCount = 0;
+
                         // UC5: while the user is paused, the playhead is frozen so forward lookahead
                         // never shrinks and the poll would spin indefinitely. Wait here until playback
                         // resumes (IsPaused clears) OR the fill drains on its own. Cancellation is
@@ -495,8 +527,23 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
                         while (IsPaused || await _audioInterop.IsProductionPaused(PlayerId))
                         {
                             cancellationToken.ThrowIfCancellationRequested();
+                            // [BP-DIAG] Phase 21.4 — heartbeat every ~1 s (10 × 100 ms) so a held poll
+                            // is visible without flooding; shows the loop is genuinely parked. TEMPORARY.
+                            if (bpDiagPollCount % 10 == 0)
+                            {
+                                _logger.LogInformation(
+                                    "[BP-DIAG] HOLD pause-poll iter={Iter} isPaused={IsPaused}",
+                                    bpDiagPollCount, IsPaused);
+                            }
+                            bpDiagPollCount++;
                             await Task.Delay(BackpressurePollMs, cancellationToken);
                         }
+
+                        // [BP-DIAG] Phase 21.4 — the read loop is EXITING the pause-poll and resuming
+                        // ReadAsync: the fill drained below low-water. TEMPORARY.
+                        _logger.LogInformation(
+                            "[BP-DIAG] EXIT pause-poll at chunk #{Chunk} after {Iters} polls",
+                            bpDiagChunkIndex, bpDiagPollCount);
                     }
                 }
             } while (currentBytes > 0);