feat(studio): #156 #157 mockup desktop and font resolver

Create AGENTS.md as a thin bridge to CLAUDE.md, fix isolated dat-layout previews by normalizing panel roots to the studio canvas, add a --mockup mode that mounts production-backed UI windows into one UiHost, and thread the per-element dat-font resolver through GameWindow's live imports.

Verified with dotnet test and headless studio screenshots for inventory and the mockup desktop.

tests/AcDream.App.Tests/Studio/StudioWindowTests.cs

@@ -0,0 +1,39 @@
+using AcDream.App.Studio;
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.Studio;
+
+public class StudioWindowTests
+{
+    [Fact]
+    public void ParseMockup_doesNotDefaultToSinglePanelLayout()
+    {
+        var opts = StudioOptions.Parse(new[] { @"C:\fake-dats", "--mockup" });
+
+        Assert.True(opts.Mockup);
+        Assert.Null(opts.LayoutId);
+        Assert.Null(opts.DumpSlug);
+        Assert.Null(opts.MarkupPath);
+    }
+
+    [Fact]
+    public void NormalizeSinglePanelRoot_movesDatPanelToCanvasOrigin()
+    {
+        var root = new UiPanel
+        {
+            Left = 500f,
+            Top = 138f,
+            Width = 300f,
+            Height = 362f,
+            Anchors = AnchorEdges.Left | AnchorEdges.Top | AnchorEdges.Right,
+        };
+
+        StudioWindow.NormalizeSinglePanelRoot(root);
+
+        Assert.Equal(0f, root.Left);
+        Assert.Equal(0f, root.Top);
+        Assert.Equal(300f, root.Width);
+        Assert.Equal(362f, root.Height);
+        Assert.Equal(AnchorEdges.None, root.Anchors);
+    }
+}