fix(ui): D.5.3/B.1 — dragging a toolbar item moved the window instead of the item

Found at visual verification: an occupied UiItemSlot sits inside the Draggable
toolbar frame (UiNineSlicePanel.Draggable=true), so UiRoot.OnMouseDown's FindWindow
returned the frame and the window-move branch won — press+drag on a slot moved the
whole bar instead of picking up the item. The slot wasn't CapturesPointerDrag (that
path is for self-driven text-selection and suppresses the BeginDrag promotion), and
UiRoot had no path for "a drag-source inside a draggable window."

Fix: add UiElement.IsDragSource (virtual, default false); UiItemSlot overrides it to
`ItemId != 0` (occupancy-gated). UiRoot.OnMouseDown now prioritizes IsDragSource over
window-move — an OCCUPIED slot starts the item drag (promotes to BeginDrag on >3px),
an EMPTY slot falls through to the IA-12 whole-window-drag so the bar stays movable
by its empty cells / chrome. UiRoot stays item-agnostic (reads only the bool). This
REDUCES divergence (occupied cells now drag like retail) within IA-12's umbrella — no
new register row.

Regression tests reproduce the LIVE topology (slot inside a Draggable frame); the
earlier RootWithBoundSlot tests put the slot directly under the root, so they could
not catch it. Full suite 493 pass / 0 fail / 2 skip.

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

src/AcDream.App/UI/UiElement.cs

@@ -113,6 +113,14 @@ public abstract class UiElement
     /// drag-drop candidacy is suppressed in favour of the element's own handling.</summary>
     public bool CapturesPointerDrag { get; set; }
 
+    /// <summary>If true, a left-press-and-move on this element starts a DRAG-DROP
+    /// (<see cref="UiRoot"/> promotes to BeginDrag) rather than moving a Draggable
+    /// ancestor window — so an item cell inside the toolbar frame drags the item, not
+    /// the window. Distinct from <see cref="CapturesPointerDrag"/> (a self-driven
+    /// interior drag like text selection, which does NOT promote to BeginDrag). Default
+    /// false; overridden by drag sources (e.g. an occupied <see cref="UiItemSlot"/>).</summary>
+    public virtual bool IsDragSource => false;
+
     /// <summary>Minimum size enforced while resizing.</summary>
     public float MinWidth { get; set; } = 40f;
     public float MinHeight { get; set; } = 40f;

src/AcDream.App/UI/UiItemSlot.cs

@@ -68,6 +68,14 @@ public sealed class UiItemSlot : UiElement
     public override (uint tex, int w, int h)? GetDragGhost()
         => ItemId != 0 && IconTexture != 0 ? (IconTexture, (int)Width, (int)Height) : null;
 
+    /// <summary>An OCCUPIED slot is a drag source — a press-and-move picks up the item
+    /// rather than moving the toolbar window. An EMPTY slot is NOT a drag source, so a
+    /// press-and-move there falls through to the IA-12 whole-window-drag, keeping the bar
+    /// movable by its empty cells / chrome. Drives <see cref="UiRoot"/>'s mousedown
+    /// window-vs-item disambiguation (retail moves the window via a dragbar, never cells;
+    /// our whole-window-drag approximation reconciles by gating on occupancy).</summary>
+    public override bool IsDragSource => ItemId != 0;
+
     /// <summary>Walk up to the containing <see cref="UiItemList"/> (the drop handler owner).</summary>
     private UiItemList? FindList()
     {

src/AcDream.App/UI/UiRoot.cs

@@ -260,6 +260,17 @@ public sealed class UiRoot : UiElement
                 _resizeMouseX = x; _resizeMouseY = y;
                 _dragCandidate = false;
             }
+            else if (target.IsDragSource)
+            {
+                // A drag SOURCE (e.g. an occupied item cell) inside a Draggable window
+                // starts an item drag-drop, NOT a window move. UiRoot stays item-agnostic:
+                // it only reads the IsDragSource flag (the cell decides occupancy). The
+                // BeginDrag promotion happens on the >3px move (and cancels if the source's
+                // GetDragPayload() returns null). Empty cells are NOT drag sources, so they
+                // fall through to window.Draggable below (IA-12 whole-window-drag), keeping
+                // the bar movable by its empty cells / chrome.
+                _dragCandidate = true;
+            }
             else if (target.CapturesPointerDrag)
             {
                 // The pressed widget owns interior drags (e.g. text selection):