refactor(inventory): consolidate IsCurrency, skip num=0 grants in history

- Drop IsWalletCurrency (duplicate of IsCurrency); use IsCurrency in WriteAcquireHistory.
- Add comment on first SaveChangesAsync in CommitAsync explaining the two-phase flush.
- Guard WriteAcquireHistory loop with grant.Num == 0 check so synthetic DebitItem post-state ops do not produce history rows.
- Add InventoryHistoryTests.Commit_writes_no_history_row_for_item_debit to lock in the fix.

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

SVSim.Database/Services/Inventory/InventoryTransaction.cs

@@ -276,6 +276,7 @@ internal sealed class InventoryTransaction : IInventoryTransaction
     {
         ThrowIfCommitted();
 
+        // Flush entity mutations first so audit-history rows are staged on top of post-commit state.
         await _db.SaveChangesAsync(ct);
 
         WriteAcquireHistory();
@@ -298,10 +299,11 @@ internal sealed class InventoryTransaction : IInventoryTransaction
         foreach (var op in _ops)
         {
             if (op is not GrantOp grant) continue;
+            if (grant.Num == 0) continue;  // skip synthetic post-state grants (e.g. DebitItem)
 
             var rowSource = grant.IsCascade ? GrantSource.CardCosmeticCascade : _source;
             var rowMessage = grant.IsCascade ? cascadeMessage : primaryMessage;
-            var detailId = IsWalletCurrency(grant.Type) ? 0L : grant.DetailId;
+            var detailId = IsCurrency(grant.Type) ? 0L : grant.DetailId;
 
             _db.ViewerAcquireHistory.Add(new ViewerAcquireHistoryEntry
             {
@@ -316,12 +318,6 @@ internal sealed class InventoryTransaction : IInventoryTransaction
         }
     }
 
-    private static bool IsWalletCurrency(UserGoodsType type) => type
-        is UserGoodsType.Crystal
-        or UserGoodsType.Rupy
-        or UserGoodsType.RedEther
-        or UserGoodsType.SpotCardPoint;
-
     private IReadOnlyList<GrantedReward> BuildRewardList()
     {
         // Pass 1 — for each currency type, find the last op (spend OR grant) that touched it