audio: widen forward decode cushion 30/15->60/30s + add [BP-DIAG] back-pressure instrumentation

Byte cap (96MB) unchanged as the hard OOM bound; the wider time window only lets sparse Opus use existing memory headroom to ride out decode jitter. Diag logs pin whether the block is back-pressure or decode throughput.

DeepDrftPublic/Interop/audio/OpusStreamDecoder.ts

@@ -163,6 +163,11 @@ export class OpusStreamDecoder implements IStreamingDecoder {
         // order-sensitive). configure() is deferred too — no need to spin up the decoder while
         // throttled. The C# loop also stops reading above high-water, so the stash stays small.
         if (this.isSchedulerFull?.()) {
+            // [BP-DIAG] First stash since last drain — production is now throttled and decode is parked.
+            // Trivially removable.
+            if (this.pendingBytes.length === 0) {
+                console.log('[BP-DIAG] Opus stash START (scheduler full, decode parked)');
+            }
             this.pendingBytes.push(chunk);
             return [];
         }
@@ -173,6 +178,8 @@ export class OpusStreamDecoder implements IStreamingDecoder {
         // the new chunk, through the demuxer as one contiguous feed.
         const out: AudioBuffer[] = [];
         if (this.pendingBytes.length > 0) {
+            // [BP-DIAG] Scheduler drained below low-water — replaying the stash. Trivially removable.
+            console.log(`[BP-DIAG] Opus stash DRAIN ${this.pendingBytes.length} chunks`);
             const stashed = this.pendingBytes;
             this.pendingBytes = [];
             for (const bytes of stashed) {
@@ -375,6 +382,13 @@ export class OpusStreamDecoder implements IStreamingDecoder {
             let iters = 0;
             const poll = () => {
                 if (!this.decoder || this.decoder.decodeQueueSize === 0 || iters >= MAX_YIELD_ITERS) {
+                    // [BP-DIAG] If we hit the 200 ms ceiling with the decode queue still non-empty, the
+                    // WebCodecs decoder is falling behind realtime (the throughput suspect for sustained
+                    // underrun — worse with HW accel off). Frequent CAP lines pin decode, not back-pressure,
+                    // as the block. Trivially removable.
+                    if (this.decoder && iters >= MAX_YIELD_ITERS && this.decoder.decodeQueueSize > 0) {
+                        console.log(`[BP-DIAG] Opus yield CAP hit, decodeQueueSize=${this.decoder.decodeQueueSize} (decoder behind realtime)`);
+                    }
                     resolve();
                     return;
                 }