feat(D.2b): wire UiHost input + moveable windows (UiRoot window-drag + WantCapture gate)

- UiElement: add Draggable flag; left-drag on a draggable element repositions
  it as a floating window instead of starting a drag-drop sequence.
- UiRoot: add WantsMouse/WantsKeyboard properties (mirrors ImGui's WantCaptureMouse
  pattern); add FindDraggable helper; inject _windowDragTarget state machine into
  OnMouseDown/OnMouseMove/OnMouseUp so draggable windows track the pointer offset.
- UiNineSlicePanel: set Draggable=true so retail window frames are movable by default.
- GameWindow: OR _uiHost?.Root.WantsMouse|WantsKeyboard into the SilkMouseSource
  wantCaptureMouse/wantCaptureKeyboard delegates and the direct MouseMove gate so
  game actions (movement, world-pick) are suppressed while the pointer is over a
  retail window — no double-handling with the InputDispatcher.
- GameWindow: wire all Silk Mice/Keyboards to UiHost after construction so the
  UiRoot tree receives live input.
- Tests: 3 new UiRootInputTests covering WantsMouse hit-test, window-drag
  reposition, and non-draggable panel immobility.

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

src/AcDream.App/UI/UiElement.cs

@@ -88,6 +88,11 @@ public abstract class UiElement
     /// <summary>Painter's-algorithm z-order within siblings. Higher = on top.</summary>
     public int  ZOrder          { get; set; }
 
+    /// <summary>If true, a left-drag on this element (or a non-draggable child of
+    /// it) repositions it as a movable window. Intended for top-level panels,
+    /// whose Left/Top are screen coordinates (Root sits at the origin).</summary>
+    public bool Draggable { get; set; }
+
     // ── Tree structure ──────────────────────────────────────────────────
     public UiElement? Parent { get; private set; }
 

src/AcDream.App/UI/UiNineSlicePanel.cs

@@ -27,6 +27,7 @@ public sealed class UiNineSlicePanel : UiPanel
         _resolve = resolve;
         BackgroundColor = Vector4.Zero; // suppress the base flat-rect fill
         BorderColor = Vector4.Zero;
+        Draggable = true; // retail windows are movable
     }
 
     /// <summary>

src/AcDream.App/UI/UiRoot.cs

@@ -49,12 +49,25 @@ public sealed class UiRoot : UiElement
     /// <summary>Widget with mouse capture (during click-drag).</summary>
     public UiElement? Captured { get; private set; }
 
+    /// <summary>
+    /// True when the pointer is over a widget OR a widget holds mouse capture.
+    /// The host ORs this into the InputDispatcher's WantCaptureMouse gate so game
+    /// actions (movement, world-pick) are suppressed while the user interacts with
+    /// a retail window — mirrors ImGui's WantCaptureMouse.
+    /// </summary>
+    public bool WantsMouse => Captured is not null || HitTestTopDown(MouseX, MouseY).element is not null;
+
+    /// <summary>True when a widget holds keyboard focus (e.g. a focused chat input).</summary>
+    public bool WantsKeyboard => KeyboardFocus is not null;
+
     /// <summary>Current drag source (set between drag-begin and drop/cancel).</summary>
     public UiElement? DragSource  { get; private set; }
     public object?    DragPayload { get; private set; }
     private UiElement? _lastDragHoverTarget;
     private int _pressX, _pressY;
     private bool _dragCandidate;
+    private UiElement? _windowDragTarget;
+    private int _windowDragOffX, _windowDragOffY;
     private const int DragDistanceThreshold = 3;   // pixels, retail-observed
 
     // Hover / tooltip tracking.
@@ -120,6 +133,14 @@ public sealed class UiRoot : UiElement
         MouseX = x;
         MouseY = y;
 
+        // Window-move drag takes precedence over drag-drop / hover / fall-through.
+        if (_windowDragTarget is not null)
+        {
+            _windowDragTarget.Left = x - _windowDragOffX;
+            _windowDragTarget.Top  = y - _windowDragOffY;
+            return;
+        }
+
         // If we have capture, deliver MouseMove to the captured widget
         // AND drive drag state machine; do NOT fall through.
         if (Captured is not null)
@@ -165,9 +186,22 @@ public sealed class UiRoot : UiElement
         // Set keyboard focus if target accepts it.
         if (target.AcceptsFocus) SetKeyboardFocus(target);
 
-        // Capture + arm drag candidate (drag promotes on subsequent MouseMove > threshold).
         SetCapture(target);
-        _dragCandidate = true;
+
+        // Window-move: if the target or an ancestor is Draggable, a left-drag
+        // repositions that window instead of starting a drag-drop.
+        var draggable = FindDraggable(target);
+        if (btn == UiMouseButton.Left && draggable is not null)
+        {
+            _windowDragTarget = draggable;
+            _windowDragOffX = x - (int)draggable.Left;
+            _windowDragOffY = y - (int)draggable.Top;
+            _dragCandidate = false;
+        }
+        else
+        {
+            _dragCandidate = true;
+        }
 
         // Dispatch raw MouseDown event (retail uses WM_LBUTTONDOWN = 0x201).
         int rawType = btn switch
@@ -187,6 +221,13 @@ public sealed class UiRoot : UiElement
         MouseX = x; MouseY = y;
         UpdateButtonFlag(btn, down: false);
 
+        if (_windowDragTarget is not null)
+        {
+            _windowDragTarget = null;
+            ReleaseCapture();
+            return;
+        }
+
         if (DragSource is not null)
         {
             FinishDrag(x, y);
@@ -436,6 +477,16 @@ public sealed class UiRoot : UiElement
         return (null, 0, 0);
     }
 
+    private static UiElement? FindDraggable(UiElement? e)
+    {
+        while (e is not null)
+        {
+            if (e.Draggable) return e;
+            e = e.Parent;
+        }
+        return null;
+    }
+
     private static bool ContainsAbsolute(UiElement e, int x, int y)
     {
         var sp = e.ScreenPosition;