feat(battle-node): BattleSession skeleton with EIO/SIO read pump

SVSim.BattleNode/Sessions/BattleSessionPhase.cs

@@ -0,0 +1,14 @@
+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>
+/// Where we are in the v1 scripted lifecycle. Drives which scripted frames the session pushes
+/// in response to inbound emits.
+/// </summary>
+public enum BattleSessionPhase
+{
+    AwaitingInitNetwork,
+    AwaitingLoaded,
+    AwaitingSwap,
+    AfterReady,
+    Terminal,
+}