test(core): D.5.3/B.2 — move ShortcutStoreTests to Core.Net.Tests (Rule 6) + cosmetic cleanups

- Move ShortcutStoreTests from AcDream.Core.Tests to AcDream.Core.Net.Tests (Rule 6:
  tests live in the project matching the layer under test; ShortcutStore is Core.Net)
- Replace fully-qualified System.Buffers.Binary.BinaryPrimitives. with BinaryPrimitives.
  in the two new BuildAddShortcut tests (file already has `using System.Buffers.Binary`)
- Add `using System;` to ShortcutStore.cs; change System.Array.Clear → Array.Clear
  (matches sibling file style, no behavior change)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core.Net/Items/ShortcutStore.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using AcDream.Core.Net.Messages;
 
@@ -19,7 +20,7 @@ public sealed class ShortcutStore
     /// <summary>Replace all slots from the login PlayerDescription shortcut list (item entries only).</summary>
     public void Load(IReadOnlyList<PlayerDescriptionParser.ShortcutEntry> entries)
     {
-        System.Array.Clear(_objIds);
+        Array.Clear(_objIds);
         foreach (var e in entries)
             if (e.Index < SlotCount && e.ObjectGuid != 0) _objIds[(int)e.Index] = e.ObjectGuid;
     }