feat(ui): D.2b-A — F12 toggles placeholder inventory window

UiHost RegisterWindow/ToggleWindow forwarders to UiRoot. A default-hidden
placeholder inventory window mounts in the RetailUi block + registers as
WindowNames.Inventory. OnInputAction handles the existing F12-bound
ToggleInventoryPanel action -> _uiHost.ToggleWindow (no-op when retail UI
off; gated by WantsKeyboard so it can't fire while typing in chat).

Placeholder is throwaway scaffolding; Sub-phase B swaps in the real
gmInventoryUI (0x21000023) under the same registry name.

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

src/AcDream.App/UI/UiHost.cs

@@ -103,6 +103,14 @@ public sealed class UiHost : System.IDisposable
         _                  => UiMouseButton.Left,
     };
 
+    // ── Window manager forwarders (delegate to UiRoot) ─────────────────
+
+    /// <summary>Register a top-level window for Show/Hide/Toggle. See <see cref="UiRoot.RegisterWindow"/>.</summary>
+    public void RegisterWindow(string name, UiElement window) => Root.RegisterWindow(name, window);
+
+    /// <summary>Toggle a registered window's visibility; returns the new IsVisible.</summary>
+    public bool ToggleWindow(string name) => Root.ToggleWindow(name);
+
     public void Dispose()
     {
         TextRenderer.Dispose();