refactor(core): D.5.3/B.2 — move ShortcutStore to Core.Items (decouple Load from the wire type)

Matches the ClientObjectTable model-placement convention; Load now takes (slot,objGuid) pairs so
the store has no Core.Net dependency. + self-drop wire-count assert + comment fix.

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

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

@@ -0,0 +1,39 @@
+using AcDream.Core.Items;
+using Xunit;
+
+namespace AcDream.Core.Tests.Items;
+
+public class ShortcutStoreTests
+{
+    [Fact]
+    public void Load_mapsSlotToObjId_skipsEmptyAndOutOfRange()
+    {
+        var store = new ShortcutStore();
+        store.Load(new (int, uint)[]
+        {
+            (0,  0x5001u),
+            (3,  0x5002u),
+            (5,  0u),       // empty/spell → skipped
+            (99, 0x5003u),  // out of range → skipped
+        });
+        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
+    }
+}