feat(core): D.5.3/B.2 — ShortcutStore + AddShortcut/RemoveShortcut wire + senders

ShortcutStore lands in AcDream.Core.Net.Items (not AcDream.Core.Items) because
it depends on PlayerDescriptionParser.ShortcutEntry; placing it in AcDream.Core
would create a circular dependency (Core.Net already references Core). The test
lives in AcDream.Core.Tests which gets Core.Net transitively via App.

BuildAddShortcut signature corrected from the old (seq, slotIndex, objectType,
targetId) 4×u32 layout to the retail ShortCutData wire format confirmed in the
action-bar deep-dive: Index(u32), ObjectId(u32), SpellId(u16), Layer(u16).
The old BuildAddShortcut_ThreeFields test is replaced by two new tests that
verify both item and spell shortcut packing.

WorldSession gains SendAddShortcut / SendRemoveShortcut following the
SendChangeCombatMode sender pattern.

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

tests/AcDream.Core.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.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
+    }
+}