fix(battle-node): collision-safe battle-id registration + viewer eviction

RegisterPending → TryRegisterPending (TryAdd instead of indexer) so
battle-id collisions return false instead of silently evicting a live
battle. MatchingBridge retries with fresh IDs on collision (max 5).

Before registering, EvictStaleForViewer removes any stale pending
battle the viewer left behind, enforcing the one-pending-per-viewer
invariant that was previously comment-asserted.

Store tests switched to per-test local stores to fix a race under
the assembly-wide ParallelScope.All.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/IBattleSessionStore.cs

@@ -2,8 +2,10 @@ namespace SVSim.BattleNode.Sessions;
 
 public interface IBattleSessionStore
 {
-    /// <summary>Register a battle minted by the matching bridge, awaiting a WS connect.</summary>
-    void RegisterPending(PendingBattle battle);
+    /// <summary>Register a battle minted by the matching bridge, awaiting a WS connect.
+    /// Returns false if a battle with the same id is already pending (caller should retry
+    /// with a fresh id).</summary>
+    bool TryRegisterPending(PendingBattle battle);
 
     /// <summary>Look up the pending battle. Returns null if not present.</summary>
     PendingBattle? TryGetPending(string battleId);

SVSim.BattleNode/Sessions/InMemoryBattleSessionStore.cs

@@ -6,8 +6,8 @@ public sealed class InMemoryBattleSessionStore : IBattleSessionStore
 {
     private readonly ConcurrentDictionary<string, PendingBattle> _pending = new();
 
-    public void RegisterPending(PendingBattle battle) =>
-        _pending[battle.BattleId] = battle;
+    public bool TryRegisterPending(PendingBattle battle) =>
+        _pending.TryAdd(battle.BattleId, battle);
 
     public PendingBattle? TryGetPending(string battleId) =>
         _pending.TryGetValue(battleId, out var b) ? b : null;