fix(audience): volatile _disposed + tighter shutdown ordering

Addresses cursor[bot] review comment on EventQueue.cs:29.

- Mark _disposed as volatile so the shutdown signal is visible across
  threads. On weakly-ordered architectures (ARM — Apple Silicon, etc.)
  the C# memory model does not guarantee a non-volatile write is
  eventually observed by other threads without a fence.

- Reorder Shutdown: set _disposed=true first, before canceling the drain
  thread. The shutdown flag being the first observable state change is a
  safer ordering and makes the lifecycle easier to reason about.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/Transport/EventQueue.cs

@@ -26,7 +26,8 @@ internal sealed class EventQueue : IDisposable
         private readonly Thread _drainThread;
         private readonly ManualResetEventSlim _flushGate = new ManualResetEventSlim(false);
 
-        private bool _disposed;
+        // Volatile so all threads see the shutdown signal immediately.
+        private volatile bool _disposed;
 
         /// <param name="store">Pre-created <see cref="DiskStore"/> for this queue.</param>
         /// <param name="flushIntervalSeconds">How often to drain to disk regardless of batch size.</param>
@@ -73,11 +74,18 @@ internal void FlushAsync()
         internal void Shutdown()
         {
             if (_disposed) return;
+
+            // Stop accepting new events first — closes the race window where
+            // events enqueued between Cancel and final drain would be lost.
+            _disposed = true;
+
+            // Signal the drain thread to exit, then wait for it.
             _cts.Cancel();
-            _flushGate.Set(); // Wake drain thread so it exits promptly
+            _flushGate.Set();
             _drainThread.Join(TimeSpan.FromSeconds(5));
-            DrainMemoryToDisk(); // Final drain after thread stops
-            _disposed = true;
+
+            // Final drain: anything enqueued before _disposed was set.
+            DrainMemoryToDisk();
         }
 
         // -----------------------------------------------------------------