feat(battlenode): SessionBattleEngine skeleton + types (Phase 2 N0)

SessionContentsCreator mirrors the test HeadlessContentsCreator fully (all
IBattleMgrContentsCreator members) so it compiles; Setup/Receive throw pending
the Task 3/4 probes. New files use the 'engine' extern alias.

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

SVSim.BattleEngine.Tests/SessionEngine/SessionEngineConstructionTests.cs

@@ -0,0 +1,17 @@
+using System.Linq;
+using NUnit.Framework;
+using SVSim.BattleNode.Sessions.Engine;
+
+namespace SVSim.BattleEngine.Tests.SessionEngine
+{
+    [TestFixture]
+    public class SessionEngineConstructionTests
+    {
+        [Test]
+        public void SessionBattleEngine_instantiates_and_is_not_ready_before_setup()
+        {
+            var engine = new SessionBattleEngine();
+            Assert.That(engine.IsReady, Is.False);
+        }
+    }
+}