fix: AC9 seek fine re-sync + deterministic decoder drain (WebCodecs Opus)

Seek now trims the lead-in so playback lands at the requested time, not the page start; decoder drain polls decodeQueueSize (bounded) instead of a single timeout. Minor cleanups.

DeepDrftPublic/Interop/audio/OpusSidecar.ts

@@ -123,21 +123,40 @@ export function presentationTimeSeconds(granulePosition: number, preSkip: number
     return Math.max(0, (granulePosition - preSkip) / OPUS_SAMPLE_RATE);
 }
 
+/**
+ * Result of resolving a seek time to a page-start byte offset.
+ * `byteOffset` is the Range request origin; `landingTimeSeconds` is the actual presentation time of that
+ * page (t_page ≤ positionSeconds). The caller uses the delta `positionSeconds - landingTimeSeconds` to
+ * trim the decoded leading frames so playback lands at the requested position, not at t_page (AC9).
+ */
+export interface OpusSeekResolution {
+    /** Page-start byte offset to use as the Range request origin (Ogg-sync-aligned). */
+    byteOffset: number;
+    /**
+     * Presentation time of the resolved index page (seconds). Always ≤ positionSeconds. The decoder
+     * must trim `(positionSeconds - landingTimeSeconds) * OPUS_SAMPLE_RATE` leading frames so the
+     * audible start and the reported clock both land at positionSeconds, not at landingTimeSeconds.
+     */
+    landingTimeSeconds: number;
+}
+
 /**
  * Resolve a seek time (seconds) to a file-absolute, page-start byte offset via the precomputed index —
  * the accurate VBR-safe transfer function (§3.4a A/C). Binary-searches for the largest entry whose
- * presentation time is <= `positionSeconds` and returns its exact page-start byte offset. NOT
- * interpolation, NOT byteRate math. With an empty index it degrades to the start of audio (the offset
- * of the first audio page == the setup-header length, since the server emits [setup pages][audio pages]).
+ * presentation time is <= `positionSeconds`. Returns both the page-start byte offset AND the actual
+ * landing time of that page, so callers can trim leading decoded frames to land precisely at
+ * `positionSeconds` (AC9 fine re-sync). NOT interpolation, NOT byteRate math.
+ *
+ * With an empty index it degrades to the start of audio (offset == setup-header length, landing == 0).
  *
  * This is the single source of truth for Opus seek-offset math, shared by the seek-beyond-buffer path
  * (AudioPlayer) and any byte-offset resolver. The Range fetch from this offset lands the decoder
  * Ogg-sync-aligned because every indexed offset is a real page start.
  */
-export function resolveOpusByteOffset(sidecar: OpusSeekData, positionSeconds: number): number {
+export function resolveOpusByteOffset(sidecar: OpusSeekData, positionSeconds: number): OpusSeekResolution {
     const points = sidecar.points;
     if (points.length === 0) {
-        return sidecar.setupHeaderBytes.length;
+        return { byteOffset: sidecar.setupHeaderBytes.length, landingTimeSeconds: 0 };
     }
 
     let lo = 0;
@@ -153,7 +172,10 @@ export function resolveOpusByteOffset(sidecar: OpusSeekData, positionSeconds: nu
             hi = mid - 1;
         }
     }
-    return points[best].byteOffset;
+    return {
+        byteOffset: points[best].byteOffset,
+        landingTimeSeconds: presentationTimeSeconds(points[best].granulePosition, sidecar.preSkip)
+    };
 }
 
 function toUint8Array(input: Uint8Array | ArrayBuffer | ArrayBufferView): Uint8Array {