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

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -30,6 +30,18 @@ public sealed class BattleSession
     public IBattleParticipant B { get; }
     public BattleSessionPhase Phase => _state.SessionPhase;
 
+    // Per-URI handlers consulted before the legacy switch. Populated incrementally (Tasks 5-14);
+    // each registered URI is served by its handler and its legacy switch arm goes dead.
+    private static readonly IReadOnlyDictionary<NetworkBattleUri, IFrameHandler> Handlers =
+        new Dictionary<NetworkBattleUri, IFrameHandler>();
+
+    private FrameDispatchContext BuildContext(IBattleParticipant from, MsgEnvelope env) =>
+        new()
+        {
+            A = A, B = B, From = from, Other = ReferenceEquals(from, A) ? B : A,
+            Env = env, Type = Type, BattleId = BattleId, State = _state,
+        };
+
     public BattleSession(string battleId, BattleType type, IBattleParticipant a, IBattleParticipant b,
         ILogger<BattleSession> log)
     {
@@ -132,6 +144,10 @@ public sealed class BattleSession
     internal IReadOnlyList<DispatchRoute> ComputeFrames(
         IBattleParticipant from, MsgEnvelope env)
     {
+        if (Handlers.TryGetValue(env.Uri, out var handler))
+            return handler.Handle(BuildContext(from, env));
+
+        // --- legacy switch (shrinking; deleted in the final task) ---
         var result = new List<DispatchRoute>();
         var other = ReferenceEquals(from, A) ? B : A;
         var phaseFrom = from as IHasHandshakePhase;