feat(ui): port retail selected-item mana

Parse and route the complete 0x0264 guid/fraction/valid response, send exact 0x0263 item-mana queries, and reproduce retail meter visibility plus guid-zero cancellation for mana and health selection changes. Keep the behavior in Core state and the retained selected-object controller with wire/state/UI conformance coverage.

Co-Authored-By: Codex <codex@openai.com>

src/AcDream.Core/Items/ItemManaState.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Collections.Concurrent;
+
+namespace AcDream.Core.Items;
+
+/// <summary>
+/// Last server-reported mana fraction for queried inventory items.
+/// Retail routes <c>QueryItemManaResponse (0x0264)</c> through
+/// <c>ClientUISystem::Handle_Item__QueryItemManaResponse @ 0x00563FE0</c>, including
+/// the response's validity flag, before notifying the selected-object toolbar.
+/// </summary>
+public sealed class ItemManaState
+{
+    private readonly ConcurrentDictionary<uint, float> _manaByGuid = new();
+
+    /// <summary>Fires for every valid or invalid query response.</summary>
+    public event Action<uint /*guid*/, float /*fraction*/, bool /*valid*/>? ItemManaChanged;
+
+    public float GetManaPercent(uint guid) =>
+        _manaByGuid.TryGetValue(guid, out float percent) ? percent : 0f;
+
+    public bool HasMana(uint guid) => _manaByGuid.ContainsKey(guid);
+
+    public void OnQueryItemManaResponse(uint itemGuid, float manaPercent, bool valid)
+    {
+        if (valid)
+            _manaByGuid[itemGuid] = manaPercent;
+        else
+            _manaByGuid.TryRemove(itemGuid, out _);
+
+        ItemManaChanged?.Invoke(itemGuid, manaPercent, valid);
+    }
+
+    public void Clear() => _manaByGuid.Clear();
+}