feat(battle-node): add RealParticipant.Phase for per-side handshake state

Internal setter; defaults to AwaitingInitNetwork. PvP needs A and B to
progress through the handshake states independently, which the
session-level BattleSession.Phase can't model. Session migration to read
realFrom.Phase is the next task.

SVSim.BattleNode/Sessions/Participants/RealParticipant.cs

@@ -28,6 +28,13 @@ public sealed class RealParticipant : IBattleParticipant
     public InboundTracker Inbound { get; } = new();
     public OutboundSequencer Outbound { get; } = new();
 
+    /// <summary>Per-side handshake progression. Session reads this when gating
+    /// handshake-phase synthesis (Matched / BattleStart / Deal / Swap response /
+    /// Ready). Session transitions via the setter after dispatch. Defaults to
+    /// AwaitingInitNetwork; only RealParticipant tracks this — bots have no phase
+    /// because they never send the gating URIs.</summary>
+    internal BattleSessionPhase Phase { get; set; } = BattleSessionPhase.AwaitingInitNetwork;
+
     public event Func<MsgEnvelope, CancellationToken, Task>? FrameEmitted;
 
     public RealParticipant(WebSocket ws, long viewerId, MatchContext context,