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.test.ts

@@ -253,6 +253,78 @@ test('eviction does not drop buffers under live sources or past the schedule cur
     assertEqual(dropped, 0, 'no eviction while front sources are live');
 });
 
+// handleSourceEnded cascade: eviction fires from the real production trigger (onended), not
+// via a direct evictPlayedBuffers() call. Confirms the anchor/index invariants hold end-to-end
+// through the scheduler's own event handling while playback is still active with a live source.
+//
+// Setup: 0.3s buffers so the 500ms lookahead window fits exactly two sources after
+// playFromPosition(0). Buffer 0 ends at ~0.31s, buffer 1 ends at ~0.61s — both are scheduled.
+// Clock is then advanced to t=0.6 so buffer 0's end (0.31) < evictBefore (0.6) while the live
+// source on buffer 1 pins firstLiveIndex=1, blocking further eviction. This is the mid-array
+// pinning scenario that later waves (21.2/21.3) build on.
+test('eviction via handleSourceEnded: position exact, live bufferIndex decremented, frontier respected', () => {
+    const cm = new FakeContextManager();
+    const s = makeScheduler(cm);
+    // Retain nothing behind the playhead — evict aggressively so the cascade fires.
+    s.setBackRetainSeconds(0);
+
+    // Eight 0.3s buffers. scheduleBuffersFrom with lookaheadTarget=0.5s at t=0:
+    //   after buf 0: nextScheduleTime≈0.31, lookahead=0.31 < 0.5 → continues
+    //   after buf 1: nextScheduleTime≈0.61, lookahead=0.61 > 0.5 → breaks
+    // → exactly two sources are live after playFromPosition.
+    for (let i = 0; i < 8; i++) s.addBuffer(buf(0.3));
+
+    cm.now = 0;
+    s.playFromPosition(0);
+
+    // Reach inside to see which sources were scheduled and what bufferIndex they hold.
+    const priv = s as unknown as {
+        scheduledSources: Array<{ source: FakeSource; bufferIndex: number; startTime: number; endTime: number }>;
+        nextBufferIndex: number;
+    };
+
+    // Confirm two sources are live — the setup guarantee.
+    if (priv.scheduledSources.length < 2) {
+        throw new Error(`Expected ≥2 scheduled sources after playFromPosition, got ${priv.scheduledSources.length}`);
+    }
+
+    // Identify the first and second scheduled sources by bufferIndex order.
+    const sorted = [...priv.scheduledSources].sort((a, b) => a.bufferIndex - b.bufferIndex);
+    const firstScheduled  = sorted[0]; // bufferIndex 0
+    const secondScheduled = sorted[1]; // bufferIndex 1
+    const secondBufferIndexBefore = secondScheduled.bufferIndex; // must be 1
+
+    // Record the second FakeSource so we can assert it was not stopped by eviction.
+    const secondFakeSource = secondScheduled.source as unknown as FakeSource;
+
+    // Advance clock to 0.6s. Buffer 0 ends at ~0.31s → evictBefore=0.6, end=0.31 ≤ 0.6 →
+    // droppable. Buffer 1 ends at ~0.61s → its live source pins firstLiveIndex=1 → NOT dropped.
+    cm.now = 0.6;
+
+    // Confirm playback is still active before firing the cascade.
+    assertEqual(s.isActive(), true, 'isActive must be true before cascade');
+
+    // Fire the cascade via the production trigger: stop the first source, which calls onended,
+    // which calls handleSourceEnded, which calls evictPlayedBuffers internally.
+    (firstScheduled.source as unknown as FakeSource).stop();
+
+    // (a) Absolute position must remain exactly 0.6.
+    assertClose(s.getCurrentPosition(), 0.6, 'position after handleSourceEnded cascade');
+
+    // (b) The second live source's bufferIndex must have been decremented by 1 (the one evicted
+    //     front buffer), shifting it from absolute index 1 to absolute index 0.
+    const expectedSecondIndex = secondBufferIndexBefore - 1;
+    assertEqual(secondScheduled.bufferIndex, expectedSecondIndex, 'live source bufferIndex decremented');
+
+    // (c) Eviction stopped at firstLiveIndex=1, not nextBufferIndex — the second buffer was
+    //     NOT dropped. Verify the second source was not stopped (it remained live throughout).
+    assertEqual(secondFakeSource.stopped, false, 'live second source not stopped by eviction');
+    // And the scheduler still has buffers (the array was not wiped past the frontier).
+    if (s.getBufferCount() === 0) {
+        throw new Error('eviction wiped all buffers — should have stopped at firstLiveIndex');
+    }
+});
+
 // --- run -------------------------------------------------------------------------------------
 if (failures.length > 0) {
     console.error(failures.join('\n'));