fix(battle-node): serialize per-session dispatch to stop cross-thread state race

In PvP a BattleSession subscribes to both participants' FrameEmitted, and each
RealParticipant raises it from its own WebSocket read loop -- two threads. The
dispatch path (ComputeFrames + the relay PushAsync calls) mutates shared,
non-thread-safe state: the BattleSessionState dictionaries (deck maps, post-swap
hands, idx->cardId reveal map). Concurrent frames from both players could corrupt
those dictionaries (InvalidOperationException / torn playSeq / wrong card identity).

Add a per-session SemaphoreSlim _dispatchGate around the whole HandleFrameAsync so
both read loops funnel through one critical section. ComputeFrames stays lock-free
(the direct-call test seam is single-threaded).

Analysis during the fix showed each OutboundSequencer is single-writer-per-instance
in steady state (A's loop only writes B's Outbound and vice-versa), so the live race
is the shared BattleSessionState, which the gate fully serializes.

TDD: BattleSessionDispatchConcurrencyTests drives both participants to AfterReady,
then fires TurnStart from both at once; the target PushAsync records peak in-flight
dispatches. Red (MaxConcurrent=2) before the gate, green (1) after.

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

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -22,6 +22,13 @@ public sealed class BattleSession
 
     private readonly BattleSessionState _state = new();
 
+    /// <summary>Serializes dispatch. Both participants' read loops raise FrameEmitted on their own
+    /// threads, and a dispatch (<see cref="ComputeFrames"/> + the relay <c>PushAsync</c> calls) mutates
+    /// shared, non-thread-safe state — the <see cref="BattleSessionState"/> dictionaries and each
+    /// participant's <c>OutboundSequencer</c>. This gate funnels both threads through one critical
+    /// section so concurrent frames can't corrupt that state.</summary>
+    private readonly SemaphoreSlim _dispatchGate = new(1, 1);
+
     /// <summary>The per-battle master seed (see <see cref="BattleSessionState.MasterSeed"/>).
     /// Exposed for logging + future replay persistence.</summary>
     public int MasterSeed => _state.MasterSeed;
@@ -146,6 +153,7 @@ public sealed class BattleSession
 
     private async Task HandleFrameAsync(IBattleParticipant from, MsgEnvelope env, CancellationToken ct)
     {
+        await _dispatchGate.WaitAsync(ct).ConfigureAwait(false);
         try
         {
             var routes = ComputeFrames(from, env);
@@ -158,6 +166,10 @@ public sealed class BattleSession
         {
             _log.LogError(ex, "BattleSession {Bid}: unhandled in HandleFrameAsync", BattleId);
         }
+        finally
+        {
+            _dispatchGate.Release();
+        }
     }
 
     /// <summary>