NEO-146: fix in-memory outcome idempotency race and clear parity

Lock idempotency keys alongside row ids in TryAppend so concurrent
duplicate keys cannot both succeed; return true from TryClearInstance
when the row is absent to match Postgres idempotent DELETE behavior.

server/NeonSprawl.Server.Tests/Game/Contracts/InMemoryContractInstanceStoreTests.cs

@@ -265,6 +265,18 @@ public sealed class InMemoryContractInstanceStoreTests
         }
     }
 
+    [Fact]
+    public void TryClearInstance_ShouldReturnTrue_WhenRowMissing()
+    {
+        // Arrange
+        var store = CreateStore();
+        // Act
+        var cleared = store.TryClearInstance(PlayerId, InstanceId);
+        // Assert
+        Assert.True(cleared);
+        Assert.False(store.TryGet(PlayerId, InstanceId, out _));
+    }
+
     [Fact]
     public async Task Host_ShouldResolveContractInstanceStoresFromDi()
     {

server/NeonSprawl.Server.Tests/Game/Contracts/InMemoryContractOutcomeStoreTests.cs

@@ -107,6 +107,29 @@ public sealed class InMemoryContractOutcomeStoreTests
         Assert.Empty(store.GetOutcomesForInstance(InstanceId));
     }
 
+    [Fact]
+    public async Task TryAppend_ShouldAllowOnlyOneSuccess_WhenConcurrentDuplicateIdempotencyKeysRace()
+    {
+        // Arrange
+        var (instanceStore, store) = CreateStores();
+        SeedActiveInstance(instanceStore);
+        var idempotencyKey = ContractOutcomeIds.MakeIdempotencyKey(PlayerId, InstanceId);
+        var baseRow = CreatePrototypeOutcome();
+        // Act
+        var results = await Task.WhenAll(
+            Enumerable.Range(0, 8).Select(i => Task.Run(() =>
+            {
+                var row = baseRow with { Id = $"outcome-row-race-{i:D3}" };
+                Assert.Equal(idempotencyKey, row.IdempotencyKey);
+                return store.TryAppend(row);
+            })));
+
+        // Assert
+        Assert.Equal(1, results.Count(static r => r));
+        Assert.Equal(7, results.Count(static r => !r));
+        Assert.Single(store.GetOutcomesForInstance(InstanceId));
+    }
+
     private static (InMemoryContractInstanceStore InstanceStore, InMemoryContractOutcomeStore OutcomeStore) CreateStores()
     {
         var options = Options.Create(new GamePositionOptions { DevPlayerId = PlayerId });

server/NeonSprawl.Server/Game/Contracts/InMemoryContractInstanceStore.cs

@@ -263,7 +263,7 @@ public sealed class InMemoryContractInstanceStore(IOptions<GamePositionOptions>
                 if (!byInstanceId.TryGetValue(instanceId, out var row) ||
                     !string.Equals(row.PlayerId, player, StringComparison.Ordinal))
                 {
-                    return false;
+                    return true;
                 }
 
                 _ = byInstanceId.TryRemove(instanceId, out _);

server/NeonSprawl.Server/Game/Contracts/InMemoryContractOutcomeStore.cs

@@ -25,17 +25,47 @@ public sealed class InMemoryContractOutcomeStore(IContractInstanceStore instance
             return false;
         }
 
-        lock (idLocks.GetOrAdd(normalized.Id, _ => new object()))
+        var appended = false;
+        LockRowAndIdempotencyKey(normalized.Id, normalized.IdempotencyKey, () =>
         {
             if (rowsById.ContainsKey(normalized.Id) ||
                 rowIdByIdempotencyKey.ContainsKey(normalized.IdempotencyKey))
             {
-                return false;
+                return;
             }
 
             rowsById[normalized.Id] = normalized;
             rowIdByIdempotencyKey[normalized.IdempotencyKey] = normalized.Id;
-            return true;
+            appended = true;
+        });
+
+        return appended;
+    }
+
+    private static string IdempotencyLockKey(string idempotencyKey) => $"idem\0{idempotencyKey}";
+
+    private void LockRowAndIdempotencyKey(string rowId, string idempotencyKey, Action action)
+    {
+        var idemLockKey = IdempotencyLockKey(idempotencyKey);
+        if (string.CompareOrdinal(rowId, idemLockKey) <= 0)
+        {
+            lock (idLocks.GetOrAdd(rowId, _ => new object()))
+            {
+                lock (idLocks.GetOrAdd(idemLockKey, _ => new object()))
+                {
+                    action();
+                }
+            }
+
+            return;
+        }
+
+        lock (idLocks.GetOrAdd(idemLockKey, _ => new object()))
+        {
+            lock (idLocks.GetOrAdd(rowId, _ => new object()))
+            {
+                action();
+            }
         }
     }
 
@@ -99,6 +129,7 @@ public sealed class InMemoryContractOutcomeStore(IContractInstanceStore instance
             if (rowsById.TryRemove(id, out var removed))
             {
                 rowIdByIdempotencyKey.TryRemove(removed.IdempotencyKey, out _);
+                idLocks.TryRemove(IdempotencyLockKey(removed.IdempotencyKey), out _);
             }
 
             idLocks.TryRemove(id, out _);