Phase 21.2: back-pressure to bound the unplayed decoded region

Shared scheduler fill signal (forward water-marks + hard byte cap) pauses
the C# read loop above high-water and, for Opus, stops the demux/decode
feed so WebCodecs queues stay near-empty. Routes through the existing
cancellation discipline; releases the latch on clear/seek.

DeepDrftPublic.Client/Services/AudioInteropService.cs

@@ -159,6 +159,25 @@ public class AudioInteropService : IAsyncDisposable
         }
     }
 
+    /// <summary>
+    /// Phase 21.2a back-pressure poll: ask whether the scheduler is still over its forward
+    /// high/low-water band. The read loop calls this only WHILE already throttled, to learn when it
+    /// may resume reading — the steady-state loop reads the piggybacked <c>ProductionPaused</c> flag
+    /// off each chunk result instead. Defaults to false on any interop failure so a torn-down player
+    /// never wedges a loop that is exiting anyway.
+    /// </summary>
+    public async Task<bool> IsProductionPaused(string playerId)
+    {
+        try
+        {
+            return await _jsRuntime.InvokeAsync<bool>("DeepDrftAudio.isProductionPaused", playerId);
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
     public async Task<AudioOperationResult> ReinitializeFromOffset(string playerId, long totalStreamLength, double seekPosition)
     {
         return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.reinitializeFromOffset", playerId, totalStreamLength, seekPosition);
@@ -419,6 +438,11 @@ public class StreamingResult : AudioOperationResult
     public bool HeaderParsed { get; set; }
     public int BufferCount { get; set; }
     public double? Duration { get; set; } // Duration in seconds calculated from WAV header
+
+    // Phase 21.2a back-pressure: true when the scheduler's forward decoded fill is over the
+    // high-water mark and the C# read loop should stop calling ReadAsync until it drains. Read off
+    // the chunk result the loop already awaits — no extra interop hop in the unthrottled steady state.
+    public bool ProductionPaused { get; set; }
 }
 
 public class AudioPlayerState