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>

tests/AcDream.Core.Net.Tests/Items/ShortcutStoreTests.cs

@@ -0,0 +1,42 @@
+using System.Collections.Generic;
+using AcDream.Core.Net.Items;
+using AcDream.Core.Net.Messages;
+using Xunit;
+
+namespace AcDream.Core.Net.Tests.Items;
+
+public class ShortcutStoreTests
+{
+    [Fact]
+    public void Load_mapsIndexToObjId_skipsEmptyAndOutOfRange()
+    {
+        var store = new ShortcutStore();
+        var entries = new List<PlayerDescriptionParser.ShortcutEntry>
+        {
+            new(Index: 0,  ObjectGuid: 0x5001u, SpellId: 0, Layer: 0),
+            new(Index: 3,  ObjectGuid: 0x5002u, SpellId: 0, Layer: 0),
+            new(Index: 5,  ObjectGuid: 0u,      SpellId: 7, Layer: 1),   // spell-only → skipped (item store)
+            new(Index: 99, ObjectGuid: 0x5003u, SpellId: 0, Layer: 0),   // out of range → skipped
+        };
+        store.Load(entries);
+        Assert.Equal(0x5001u, store.Get(0));
+        Assert.Equal(0x5002u, store.Get(3));
+        Assert.Equal(0u, store.Get(5));
+        Assert.True(store.IsEmpty(1));
+    }
+
+    [Fact]
+    public void SetRemoveGet_roundtrip_andBoundsSafe()
+    {
+        var store = new ShortcutStore();
+        store.Set(4, 0xABCDu);
+        Assert.Equal(0xABCDu, store.Get(4));
+        Assert.False(store.IsEmpty(4));
+        store.Remove(4);
+        Assert.True(store.IsEmpty(4));
+        Assert.Equal(0u, store.Get(-1));
+        Assert.Equal(0u, store.Get(18));
+        store.Set(18, 1u);     // no-op, no throw
+        store.Remove(-1);      // no-op, no throw
+    }
+}