feat(dispatcher): add registration-time pipeline behavior ordering via AddBehavior<T>(order:)

Pipeline behavior ordering previously depended on discovery/registration
order. Add an AddBehavior<T>(order:) API that captures the order at
registration time; the BehaviorTypeIndex now orders matching behaviors by
that explicit order (lower runs first), falling back to registration order
for ties. Removed the runtime Order property approach.

src/request.dispatcher.tests/ScalarBehaviourTests.cs

@@ -87,6 +87,26 @@ internal sealed class ScalarBehaviourTests
 		await Assert.That(tracker.Executed).IsTrue();
 	}
 
+	[Test]
+	public async Task I_can_order_behaviours_explicitly_by_order_property()
+	{
+		var sc = new ServiceCollection();
+		sc.AddSingleton<ScalarTestTracker>();
+		sc.AddRequestDispatcher(builder => builder
+			.Add(typeof(ScalarTestHandler))
+			.AddBehavior<OrderedScalarBehaviorA>(2)
+			.AddBehavior<OrderedScalarBehaviorB>(1));
+		var provider = sc.BuildServiceProvider();
+		var dispatcher = provider.GetRequiredService<IRequestDispatcher>();
+		var tracker = provider.GetRequiredService<ScalarTestTracker>();
+
+		var request = new ScalarTestRequest { Value = "Hello" };
+		await dispatcher.DispatchAsync(request);
+
+		// Registered A (order 2) then B (order 1) via AddBehavior; explicit order overrides discovery order.
+		await Assert.That(tracker.Log).IsEquivalentTo(["OrderedB", "OrderedA"]);
+	}
+
 	[Test]
 	public async Task I_can_maintain_the_ordering_between_open_and_closed_behaviours()
 	{