feat(battlenode): Setup builds two-seat network battle headless (Phase 2 N0)

Mirrors HeadlessFixture.NewNetworkEmitBattle wiring (opponent seating, leader
life, card templates, deck seeding) minus the emit-only RealTimeNetworkAgent
scaffolding (shadow only receives). Probe passed first run — M13 already filled
the network-mgr construction gaps. No Engine/ edits; drift clean.

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

SVSim.BattleEngine.Tests/SessionEngine/SessionEngineConstructionTests.cs

@@ -13,5 +13,19 @@ namespace SVSim.BattleEngine.Tests.SessionEngine
             var engine = new SessionBattleEngine();
             Assert.That(engine.IsReady, Is.False);
         }
+
+        [Test]
+        public void Setup_builds_two_seat_network_battle_headless()
+        {
+            HeadlessEngineEnv.EnsureInitialized();
+            // Load every card id the two test decks reference so CardMaster can resolve them.
+            var deckA = Enumerable.Repeat(100011010L, 40).ToList(); // vanilla 1/2 follower x40
+            var deckB = Enumerable.Repeat(100011010L, 40).ToList();
+            HeadlessCardMaster.Load(100011010);
+
+            var engine = new SessionBattleEngine();
+            Assert.DoesNotThrow(() => engine.Setup(masterSeed: 12345, seatADeck: deckA, seatBDeck: deckB));
+            Assert.That(engine.IsReady, Is.True);
+        }
     }
 }