Reconcile eviction comment wording; add handleSourceEnded cascade test (Phase 21.1)

The inclusive <= bound is correct; comments now say 'at or behind'. New
test drives eviction through the real onended trigger with a live mid-array
source pinning the frontier.

DeepDrftPublic/Interop/audio/PlaybackScheduler.ts

@@ -136,7 +136,7 @@ export class PlaybackScheduler {
      * Drop already-played buffers from the front of the array, reclaiming their decoded float
      * memory, and advance the time anchor so all position/index bookkeeping stays exact.
      *
-     * Eviction frontier: any buffer whose absolute END time is older than
+     * Eviction frontier: any buffer whose absolute END time is at or older than
      * (currentPosition - backRetainSeconds) is droppable. We evict a contiguous run from the
      * front only — buffers are appended in playback order, so the front is always the oldest.
      *
@@ -179,7 +179,7 @@ export class PlaybackScheduler {
         let accumulatedEnd = this.playbackOffset;
         for (let i = 0; i < maxEvictable; i++) {
             accumulatedEnd += this.buffers[i].duration;
-            // Drop only buffers whose END is strictly behind the retain frontier.
+            // Drop buffers whose END is at or behind the retain frontier (inclusive bound).
             if (accumulatedEnd <= evictBefore) {
                 evictCount = i + 1;
             } else {