21.3 review fixes: guard superseded-seek failures; restore post-recovery retry

C6/AC8: IsStillActiveSeek() predicate guards all three SeekBeyondBuffer
failure exits, so a superseded seek never recovers over a newer seek's
state. AC6: empty scheduler routes to seekBeyondBuffer so a same-target
retry (seek or play) refetches instead of no-oping.

DeepDrftPublic/Interop/audio/AudioPlayer.ts

@@ -440,21 +440,28 @@ export class AudioPlayer {
         const bufferEnd = this.scheduler.getTotalDuration() + bufferStart;
 
         // The window-miss test for BOTH directions, and the 21.3 refill trigger for backward seeks.
-        // Position must be within [bufferStart, bufferEnd] to resolve from the retained buffers:
-        //   - position >= bufferStart  : UC3 — seek back within the retained back-window. Served from
-        //     buffer with NO network refetch. (The lower bound is load-bearing: after eviction or a
-        //     prior seek-beyond-buffer, bufferStart > 0, and a target below it would otherwise produce
-        //     a negative bufferRelativePosition in seekWithinBuffer, silently clamping to position 0.)
+        // Position must be within [bufferStart, bufferEnd] AND the scheduler must hold buffers to
+        // resolve from the retained window:
+        //   - position >= bufferStart AND hasBuffers : UC3 — seek back within the retained back-window.
+        //     Served from buffer with NO network refetch. (The lower bound is load-bearing: after
+        //     eviction or a prior seek-beyond-buffer, bufferStart > 0, and a target below it would
+        //     otherwise produce a negative bufferRelativePosition in seekWithinBuffer, silently clamping
+        //     to position 0.)
         //   - position < bufferStart   : UC4 — seek back PAST the retained tail (the window was evicted).
         //     Falls through to seekBeyondBuffer, which is the existing Range path run toward an EARLIER
         //     offset. This is the 21.3 window-miss refill: "a seek the listener didn't initiate" reuses
         //     the same per-path resolver + reinit a forward seek-beyond-buffer uses, no new mechanism.
         //   - position > bufferEnd     : UC2/UC5 — forward seek beyond buffer, unchanged.
-        if (position >= bufferStart && position <= bufferEnd) {
+        //   - !hasBuffers (degenerate [P,P] window post-recovery): the window check above would
+        //     spuriously route ANY target to seekWithinBuffer (bufferStart==bufferEnd==seekPosition
+        //     after recoverFromFailedRefill). Force seekBeyondBuffer so a same-target retry actually
+        //     refetches (AC6 retry contract). The !hasBuffers guard only fires in the degenerate case —
+        //     a populated retained window has buffers and is unaffected (AC4 not regressed).
+        if (position >= bufferStart && position <= bufferEnd && this.scheduler.hasBuffers()) {
             return this.seekWithinBuffer(position);
         } else {
-            // Seeking outside the retained window - signal C# to fetch a new stream from the resolved
-            // offset (earlier for a back-past-window refill, later for a forward seek).
+            // Seeking outside the retained window, or to any position in an empty scheduler —
+            // signal C# to fetch a new stream from the resolved offset.
             return this.seekBeyondBuffer(position);
         }
     }