test(battle-engine M13): NewNetworkEmitBattle harness + OnEmit capture seam

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleEngine.Tests/NetworkMgrConstructionProbeTests.cs

@@ -21,5 +21,15 @@ namespace SVSim.BattleEngine.Tests
                 Assert.That(mgr, Is.Not.Null);
             });
         }
+
+        [Test]
+        public void OnEmit_capture_seam_is_wired_via_injected_agent()
+        {
+            var (mgr, emitted) = HeadlessEngineEnv.NewNetworkEmitBattle();
+            Assert.That(mgr, Is.Not.Null);
+            Assert.That(Wizard.ToolboxGame.RealTimeNetworkAgent, Is.Not.Null,
+                "agent must be injected so NetworkBattleSender's ToolboxGame.RealTimeNetworkAgent.* calls resolve");
+            Assert.That(emitted, Is.Empty, "no emit yet — only the seam is wired");
+        }
     }
 }