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/Interop/audio/PlaybackScheduler.ts

@@ -107,6 +107,14 @@ export class PlaybackScheduler {
     // Mutated by evaluateProductionPause() — named to signal the state-advance on each call.
     private productionPaused_: boolean = false;
 
+    // ─────────────────────────────────────────────────────────────────────────────────────────
+    // [BP-DIAG] Phase 21.4 back-pressure diagnostic. TEMPORARY — strip once the cause is confirmed
+    // in Daniel's browser run. Throttles evaluateProductionPause() logging to one line per ~250 ms
+    // so the console shows the live lookahead / byte-estimate / latch without flooding (the signal
+    // is evaluated on every chunk + every poll). Grep "[BP-DIAG]" in the browser console.
+    private bpDiagLastLogMs: number = 0;
+    // ─────────────────────────────────────────────────────────────────────────────────────────
+
     // Callbacks
     public onPlaybackEnded: (() => void) | null = null;
 
@@ -237,6 +245,22 @@ export class PlaybackScheduler {
             this.productionPaused_ = true;
         }
 
+        // [BP-DIAG] Phase 21.4 — the single source of truth for the latch decision. If `paused`
+        // never goes true while bytes keep arriving, the lookahead is not growing as expected:
+        // inspect `lookahead` vs `high` (should cross 30 during a fast fill) and `bufCount`/`bytes`
+        // (decode must actually be populating the scheduler). Throttled to ~4 Hz. TEMPORARY — strip
+        // once the cause is confirmed. (Uses performance.now when present; Date.now fallback.)
+        const bpNow = (typeof performance !== 'undefined' ? performance.now() : Date.now());
+        if (bpNow - this.bpDiagLastLogMs >= 250) {
+            this.bpDiagLastLogMs = bpNow;
+            console.log(
+                `[BP-DIAG] evaluateProductionPause paused=${this.productionPaused_} ` +
+                `lookahead=${lookahead.toFixed(2)}s high=${this.forwardHighWaterSeconds} low=${this.forwardLowWaterSeconds} ` +
+                `bytes=${(this.getDecodedByteEstimate() / (1024 * 1024)).toFixed(1)}MB cap=${(this.maxDecodedBytes / (1024 * 1024)).toFixed(0)}MB ` +
+                `overByteCeiling=${overByteCeiling} bufCount=${this.buffers.length} pos=${this.getCurrentPosition().toFixed(2)}s ` +
+                `decodedEnd=${(this.getTotalDuration() + this.playbackOffset).toFixed(2)}s active=${this.isActive_}`);
+        }
+
         return this.productionPaused_;
     }