Fix netstandard2.1 build: avoid Task.WaitAsync

Task.WaitAsync(CancellationToken) is .NET 6+, but ChibiRuby also
targets netstandard2.1. Replace it with a hand-rolled WhenAny-based
wait inside WaitForTerminateAsync so all four TFMs compile.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hadashiA

src/ChibiRuby/RFiber.cs

@@ -53,11 +53,31 @@ public ValueTask<MRubyValue> WaitForResumeAsync(CancellationToken cancellation =
     public ValueTask<MRubyValue> WaitForTerminateAsync(CancellationToken cancellation = default)
     {
         var task = terminationSource.Task;
-        if (cancellation.CanBeCanceled)
+        if (!cancellation.CanBeCanceled)
         {
-            task = task.WaitAsync(cancellation);
+            return new ValueTask<MRubyValue>(task);
+        }
+        return AwaitWithCancellationAsync(task, cancellation);
+
+        // Hand-rolled equivalent of Task.WaitAsync(cancellation): the latter
+        // only exists on .NET 6+, but this assembly also targets
+        // netstandard2.1.
+        static async ValueTask<MRubyValue> AwaitWithCancellationAsync(
+            Task<MRubyValue> task, CancellationToken cancellation)
+        {
+            var cancelTcs = new TaskCompletionSource<bool>(
+                TaskCreationOptions.RunContinuationsAsynchronously);
+            using (cancellation.Register(
+                static state => ((TaskCompletionSource<bool>)state!).TrySetResult(true),
+                cancelTcs))
+            {
+                if (await Task.WhenAny(task, cancelTcs.Task).ConfigureAwait(false) != task)
+                {
+                    cancellation.ThrowIfCancellationRequested();
+                }
+            }
+            return await task.ConfigureAwait(false);
         }
-        return new ValueTask<MRubyValue>(task);
     }
 
     public async IAsyncEnumerable<MRubyValue> AsAsyncEnumerable([EnumeratorCancellation] CancellationToken cancellation = default)