feat(ui): D.2b-A — BringToFront + raise on show/click

BringToFront sets a window's ZOrder one past the max among its peers.
ShowWindow now raises on open; OnMouseDown raises any pressed top-level
window (retail-faithful stacking). Existing drag/resize tests unaffected
(raise only touches ZOrder, not geometry).

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

src/AcDream.App/UI/UiRoot.cs

@@ -248,6 +248,8 @@ public sealed class UiRoot : UiElement
         // A left-drag starting near an edge resizes; interior drag repositions;
         // otherwise it's a normal drag-drop candidate.
         var window = FindWindow(target);
+        // Retail-faithful: pressing on a window raises it above its peers.
+        if (window is not null) BringToFront(window);
         if (btn == UiMouseButton.Left && window is not null)
         {
             var edges = window.Resizable ? HitEdges(window, x, y, ResizeGrip) : ResizeEdges.None;
@@ -489,6 +491,7 @@ public sealed class UiRoot : UiElement
     {
         if (!_windows.TryGetValue(name, out var w)) return false;
         w.Visible = true;
+        BringToFront(w);
         return true;
     }
 
@@ -510,6 +513,18 @@ public sealed class UiRoot : UiElement
         return true;
     }
 
+    /// <summary>Raise a top-level window above its siblings by setting its ZOrder
+    /// one past the current max among the OTHER top-level children. Used on Show
+    /// and on click. Leaves ZOrder unchanged if it is the only / already-topmost child.</summary>
+    public void BringToFront(UiElement window)
+    {
+        int top = window.ZOrder;
+        foreach (var c in Children)
+            if (!ReferenceEquals(c, window))
+                top = System.Math.Max(top, c.ZOrder + 1);
+        window.ZOrder = top;
+    }
+
     // ── Drag-drop (retail event chain 0x15 → 0x21 → 0x1C → 0x3E) ────────
 
     private void BeginDrag(UiElement source)

tests/AcDream.App.Tests/UI/UiRootInputTests.cs

@@ -289,4 +289,45 @@ public class UiRootInputTests
         Assert.False(root.HideWindow("nope"));
         Assert.False(root.ToggleWindow("nope"));
     }
+
+    [Fact]
+    public void ShowWindow_RaisesAbovePeers()
+    {
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var a = new UiPanel { Width = 100, Height = 100 };
+        var b = new UiPanel { Width = 100, Height = 100 };
+        var win = new UiPanel { Width = 100, Height = 100, Visible = false };
+        root.AddChild(a); root.AddChild(b); root.AddChild(win);
+        root.RegisterWindow("inventory", win);
+
+        root.ShowWindow("inventory");
+        Assert.True(win.ZOrder > a.ZOrder);
+        Assert.True(win.ZOrder > b.ZOrder);
+    }
+
+    [Fact]
+    public void BringToFront_SetsStrictlyGreatestZOrder()
+    {
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var a = new UiPanel { Width = 100, Height = 100, ZOrder = 5 };
+        var b = new UiPanel { Width = 100, Height = 100, ZOrder = 9 };
+        root.AddChild(a); root.AddChild(b);
+
+        root.BringToFront(a);
+        Assert.True(a.ZOrder > b.ZOrder);   // 10 > 9
+    }
+
+    [Fact]
+    public void MouseDown_OnWindow_RaisesItAbovePeers()
+    {
+        var root = new UiRoot { Width = 800, Height = 600 };
+        // 'peer' has a higher ZOrder but does NOT cover (50,50); 'clicked' does.
+        var peer    = new UiPanel { Left = 300, Top = 0, Width = 100, Height = 100, ZOrder = 9 };
+        var clicked = new UiPanel { Left = 0,   Top = 0, Width = 100, Height = 100, ZOrder = 0, Draggable = true };
+        root.AddChild(peer); root.AddChild(clicked);
+
+        root.OnMouseDown(UiMouseButton.Left, 50, 50);   // only 'clicked' occupies (50,50)
+        Assert.True(clicked.ZOrder > peer.ZOrder);
+        root.OnMouseUp(UiMouseButton.Left, 50, 50);
+    }
 }