feat(ui): D.2b-A — UiRoot named-window registry (Show/Hide/Toggle)

A Dictionary<string,UiElement> registry on UiRoot with RegisterWindow +
Show/Hide/Toggle. Show/Hide flip UiElement.Visible (already gates
Draw/Tick/HitTest); Toggle returns the new visibility; unknown names are
no-ops. WindowNames.Inventory const shared by mount/registry/toggle.

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

src/AcDream.App/UI/UiRoot.cs

@@ -474,6 +474,42 @@ public sealed class UiRoot : UiElement
     public void SetCapture(UiElement e) => Captured = e;
     public void ReleaseCapture() => Captured = null;
 
+    // ── Window manager (named top-level windows: Show / Hide / Toggle) ───
+
+    private readonly Dictionary<string, UiElement> _windows = new();
+
+    /// <summary>Register a top-level window under a name for Show/Hide/Toggle.
+    /// Does NOT add it to the tree — the caller mounts via AddChild and controls
+    /// initial Visible. Idempotent (re-register replaces). The dict is the source
+    /// of truth — independent of UiElement.Name (init-only, not set here).</summary>
+    public void RegisterWindow(string name, UiElement window) => _windows[name] = window;
+
+    /// <summary>Make the named window visible. No-op (returns false) if unknown.</summary>
+    public bool ShowWindow(string name)
+    {
+        if (!_windows.TryGetValue(name, out var w)) return false;
+        w.Visible = true;
+        return true;
+    }
+
+    /// <summary>Hide the named window. No-op (returns false) if unknown.</summary>
+    public bool HideWindow(string name)
+    {
+        if (!_windows.TryGetValue(name, out var w)) return false;
+        w.Visible = false;
+        return true;
+    }
+
+    /// <summary>Flip the named window's visibility (Show if hidden, Hide if shown).
+    /// Returns the new IsVisible state (false for an unknown name).</summary>
+    public bool ToggleWindow(string name)
+    {
+        if (!_windows.TryGetValue(name, out var w)) return false;
+        if (w.Visible) { HideWindow(name); return false; }
+        ShowWindow(name);
+        return true;
+    }
+
     // ── Drag-drop (retail event chain 0x15 → 0x21 → 0x1C → 0x3E) ────────
 
     private void BeginDrag(UiElement source)

src/AcDream.App/UI/WindowNames.cs

@@ -0,0 +1,8 @@
+namespace AcDream.App.UI;
+
+/// <summary>Canonical registry names for top-level retail-UI windows, so the
+/// mount, the window registry, and the toggle keybind all agree on one literal.</summary>
+public static class WindowNames
+{
+    public const string Inventory = "inventory";
+}