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/Interop/audio/index.ts

@@ -117,6 +117,14 @@ const DeepDrftAudio = {
         return player?.calculateByteOffset(positionSeconds) ?? 0;
     },
 
+    // Phase 21.2a back-pressure poll: the C# read loop calls this WHILE throttled to learn when
+    // the scheduler has drained below low-water and reading may resume. A missing player reads as
+    // "not paused" so a torn-down player never wedges a loop that is already exiting.
+    isProductionPaused: (playerId: string): boolean => {
+        const player = audioPlayers.get(playerId);
+        return player?.isProductionPaused() ?? false;
+    },
+
     reinitializeFromOffset: (playerId: string, totalStreamLength: number, seekPosition: number): AudioResult => {
         const player = audioPlayers.get(playerId);
         if (!player) return { success: false, error: 'Player not found' };