feat(ui): the spell-bar drop ring — retail's authored drag-accept state, and the ring exposed a real drop off-by-one

The green ring is retail's own art: every UIItem cell carries an
authored DragAccept child (catalog 0x21000037, child 0x1000045A), and
the spell bar's drag-over handler (SpellCastSubMenu::OnItemListDragOver
@0x004C5990) flips it to the Accept state (0x10000040 -> surface
0x060011F9) for any spell payload. Ported through a per-slot
SetDragAcceptVisual seam + a catalog DragOverAcceptance hook; other
lists are untouched (null acceptance = neutral). A polarity error in
our older docs (Accept/Reject state ids swapped) was corrected against
three independent sources; the shipped art was always right, only the
labels lied.

The ring shares ONE landing computation with the drop
(FavoriteDropIndex) — and that requirement exposed a genuine #354
off-by-one: the empty-tail path double-applied the -1 adjustment
(retail gates it on the lift's removal @0x004C7157), landing a
reordered spell second-to-last instead of last. Fixed;
discriminator-verified both ways. AP-172 narrowed + its false
empty-tail claim corrected.

Clean-room complete solution: 11,545 passed / 4 skipped / 0 failed.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/AcDream.App/UI/UiItemSlot.cs

@@ -46,10 +46,13 @@ public class UiItemSlot : UiElement
     public ItemDragSource SourceKind { get; set; } = ItemDragSource.Inventory;
 
     /// <summary>Drag-rollover accept frame (retail ItemSlot_DragOver_Accept 0x060011F9,
-    /// state id 0x10000041). Configurable; guard id != 0 before resolving.</summary>
+    /// UIStateId 0x10000040 — the state paperdoll AutoWearIsLegal @ 0x004A3AC7 and
+    /// VendorSellUI DragItemAcceptable @ 0x004C2320 set on a LEGAL target; the
+    /// 2026-06-16 deep-dive's prose swapped the 40/41 ids, the art column was right).
+    /// Configurable; guard id != 0 before resolving.</summary>
     public uint DragAcceptSprite { get; set; } = 0x060011F9u;
     /// <summary>Drag-rollover reject frame (retail ItemSlot_DragOver_Reject 0x060011F8,
-    /// state id 0x10000040).</summary>
+    /// UIStateId 0x10000041 — the illegal-target branch at 0x004A3AEB / 0x004C2336).</summary>
     public uint DragRejectSprite { get; set; } = 0x060011F8u;
 
     /// <summary>True when this cell is the OPEN container (its contents fill the grid). Draws the
@@ -85,6 +88,13 @@ public class UiItemSlot : UiElement
     /// <summary>Current overlay state — internal so unit tests can assert it (InternalsVisibleTo).</summary>
     internal DragAcceptState DragAcceptVisual => _dragAccept;
 
+    /// <summary>Set the drag-rollover overlay state — the port of retail
+    /// <c>UIElement_UIItem::SetDragAcceptState</c> @ 0x004E1290 flipping the authored
+    /// per-cell <c>m_elem_Icon_DragAccept</c> child (element 0x1000045A, bound in
+    /// <c>PostInit</c> @ 0x004E1870). Subclasses whose drops bypass
+    /// <see cref="IItemListDragHandler"/> (catalog cells) drive the same visual here.</summary>
+    private protected void SetDragAcceptVisual(DragAcceptState state) => _dragAccept = state;
+
     /// <summary>Empty-slot sprite. Default = the generic toolbar empty-slot border
     /// 0x060074CF (uiitem template 0x21000037, state ItemSlot_Empty). Configurable so
     /// paperdoll equip slots can use their per-slot silhouettes later.</summary>
@@ -445,19 +455,7 @@ public class UiItemSlot : UiElement
                 ctx.DrawSprite(tex, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
         }
 
-        // Drag-rollover accept/reject frame (retail SetDragAcceptState 0x10000041/40).
-        // Guard id != 0 BEFORE resolving — resolve(0) returns the 1×1 magenta placeholder
-        // with a non-zero GL handle (feedback_ui_resolve_zero_magenta).
-        if (_dragAccept != DragAcceptState.None && SpriteResolve is not null)
-        {
-            uint id = _dragAccept == DragAcceptState.Accept ? DragAcceptSprite : DragRejectSprite;
-            if (id != 0)
-            {
-                var (tex, _, _) = SpriteResolve(id);
-                if (tex != 0)
-                    ctx.DrawSprite(tex, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
-            }
-        }
+        DrawDragAcceptOverlay(ctx);
 
         // UIElement_UIItem::UpdateCooldownDisplay @ 0x004E1E20 selects exactly
         // one of ten authored radial overlays. The shared prototype gives these
@@ -482,6 +480,24 @@ public class UiItemSlot : UiElement
         }
     }
 
+    /// <summary>Draws the drag-rollover accept/reject frame (retail
+    /// <c>SetDragAcceptState</c> writing <c>ItemSlot_DragOver_Accept</c> 0x10000040 /
+    /// <c>_Reject</c> 0x10000041 on the authored 0x1000045A child). Shared with catalog
+    /// cells the same way retail shares the one UIItem prototype (catalog 0x21000037).
+    /// Guard id != 0 BEFORE resolving — resolve(0) returns the 1×1 magenta placeholder
+    /// with a non-zero GL handle (feedback_ui_resolve_zero_magenta).</summary>
+    protected void DrawDragAcceptOverlay(UiRenderContext ctx)
+    {
+        if (_dragAccept == DragAcceptState.None || SpriteResolve is null)
+            return;
+        uint id = _dragAccept == DragAcceptState.Accept ? DragAcceptSprite : DragRejectSprite;
+        if (id == 0)
+            return;
+        var (tex, _, _) = SpriteResolve(id);
+        if (tex != 0)
+            ctx.DrawSprite(tex, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
+    }
+
     /// <summary>Draws the shared retail shortcut-number layer. Catalog spell cells
     /// call this after their icon layer so magic favorites and physical shortcuts
     /// use the same UIItem presentation path.</summary>