NEO-54: preserve validation reason codes when player store missing

Deny helper returns Denied + reasonCode even without a snapshot bucket,
so invalid_quantity/invalid_item are stable before NEO-55 HTTP mapping.

server/NeonSprawl.Server.Tests/Game/Items/PlayerInventoryOperationsTests.cs

@@ -271,6 +271,28 @@ public sealed class PlayerInventoryOperationsTests
         Assert.Null(outcome.Snapshot);
     }
 
+    [Fact]
+    public void TryAddStack_ForUnknownPlayer_WithInvalidQuantity_ShouldDenyWithReasonCode()
+    {
+        // Arrange
+        using var factory = new InMemoryWebApplicationFactory();
+        var registry = factory.Services.GetRequiredService<IItemDefinitionRegistry>();
+        var store = factory.Services.GetRequiredService<IPlayerInventoryStore>();
+
+        // Act
+        var outcome = PlayerInventoryOperations.TryAddStack(
+            "unknown-player-xyz",
+            "scrap_metal_bulk",
+            quantity: 0,
+            registry,
+            store);
+
+        // Assert
+        Assert.Equal(PlayerInventoryMutationKind.Denied, outcome.Kind);
+        Assert.Equal(PlayerInventoryReasonCodes.InvalidQuantity, outcome.ReasonCode);
+        Assert.Null(outcome.Snapshot);
+    }
+
     private static int CountItem(PlayerInventorySnapshot snapshot, string itemId)
     {
         var total = 0;

server/NeonSprawl.Server/Game/Items/PlayerInventoryOperations.cs

@@ -124,12 +124,12 @@ public static class PlayerInventoryOperations
 
     private static PlayerInventoryMutationOutcome Deny(string playerId, IPlayerInventoryStore store, string reasonCode)
     {
-        if (!store.TryGetSnapshot(playerId, out var snapshot))
+        if (store.TryGetSnapshot(playerId, out var snapshot))
         {
-            return new PlayerInventoryMutationOutcome(PlayerInventoryMutationKind.StoreMissing, null, null);
+            return new PlayerInventoryMutationOutcome(PlayerInventoryMutationKind.Denied, reasonCode, snapshot);
         }
 
-        return new PlayerInventoryMutationOutcome(PlayerInventoryMutationKind.Denied, reasonCode, snapshot);
+        return new PlayerInventoryMutationOutcome(PlayerInventoryMutationKind.Denied, reasonCode, null);
     }
 
     private static int MergeIntoExistingStacks(InventorySlotState[] slots, string itemId, int remaining, int stackMax)