refactor(battle-node): add frame-handler contract, context, and empty registry shim

SVSim.BattleNode/Sessions/Dispatch/IFrameHandler.cs

@@ -0,0 +1,10 @@
+namespace SVSim.BattleNode.Sessions.Dispatch;
+
+/// <summary>Handles one (or more) inbound URI(s). Pure: returns the routes to dispatch and may
+/// mutate <see cref="FrameDispatchContext.State"/> / advance <see cref="FrameDispatchContext.SenderPhase"/>,
+/// but does not touch the wire. Stateless singletons live in BattleSession's registry; a single
+/// handler may be registered under multiple URIs (e.g. Retire/Kill).</summary>
+internal interface IFrameHandler
+{
+    IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx);
+}