fix(D.5.1): occupancy-gated slot numbers (empty=0x1000005e bg digit) + bottom-right rect probe

FIX 1: UIElement_UIItem::SetShortcutNum (decomp 229481) has a three-way source
branch: occupied+peace -> 0x10000042 (peace digit set), occupied+war -> 0x10000043
(war digit set), empty (ItemId==0) -> 0x1000005e (background digit, stance-independent).
acdream previously only had the peace/war pair and drew them regardless of occupancy.

Changes:
- GameWindow.cs: read property 0x1000005e into toolbarEmptyDigits (no fallback;
  null is safe). Logs entry count. Passes emptyDigits to Bind. Adds [D.5.1 probe]
  block logging screen pos + size of 7 bottom-right element ids via ScreenPosition.
- ToolbarController.cs: adds _emptyDigits field, emptyDigits ctor+Bind param (null
  default). RestampShortcutNumbers sets cell.EmptyDigits. Comments cite decomp 229481.
- UiItemSlot.cs: adds EmptyDigits property + ActiveDigitArray() internal testable seam
  (occupied -> peace/war by stance; empty -> EmptyDigits). OnDraw uses it. Comment
  updated with three-way source table.
- Tests: 5 new UiItemSlotTests (ActiveDigitArray occupancy), 2 new
  ToolbarControllerTests (emptyDigits injection + null-safe).

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

tests/AcDream.App.Tests/UI/UiItemSlotTests.cs

@@ -82,4 +82,63 @@ public class UiItemSlotTests
         s.ClearShortcutNum();
         Assert.Equal(-1, s.ShortcutNum);
     }
+
+    // ── ActiveDigitArray occupancy gating (decomp UIElement_UIItem::SetShortcutNum:229481) ──
+
+    private static readonly uint[] Peace = { 0x10u, 0x11u, 0x12u };
+    private static readonly uint[] War   = { 0x20u, 0x21u, 0x22u };
+    private static readonly uint[] Empty = { 0x30u, 0x31u, 0x32u };
+
+    /// <summary>
+    /// When ItemId == 0 (empty slot), ActiveDigitArray returns EmptyDigits regardless
+    /// of ShortcutPeace. Retail ref: UIElement_UIItem::SetShortcutNum (decomp 229481) —
+    /// else branch when m_elem_Icon->m_state == 0x1000001c (empty).
+    /// </summary>
+    [Fact]
+    public void ActiveDigitArray_emptySlot_returnsEmptyDigits()
+    {
+        var s = new UiItemSlot { PeaceDigits = Peace, WarDigits = War, EmptyDigits = Empty };
+        s.SetShortcutNum(0, peace: true);
+        // ItemId == 0 → EmptyDigits
+        Assert.Same(Empty, s.ActiveDigitArray());
+    }
+
+    [Fact]
+    public void ActiveDigitArray_emptySlot_warStance_stillReturnsEmptyDigits()
+    {
+        var s = new UiItemSlot { PeaceDigits = Peace, WarDigits = War, EmptyDigits = Empty };
+        s.SetShortcutNum(0, peace: false);
+        // ItemId == 0 → EmptyDigits regardless of stance
+        Assert.Same(Empty, s.ActiveDigitArray());
+    }
+
+    /// <summary>
+    /// When ItemId != 0 (occupied), ActiveDigitArray returns PeaceDigits or WarDigits
+    /// depending on ShortcutPeace. Retail ref: UIElement_UIItem::SetShortcutNum (decomp 229481/229493).
+    /// </summary>
+    [Fact]
+    public void ActiveDigitArray_occupiedSlot_peaceStance_returnsPeaceDigits()
+    {
+        var s = new UiItemSlot { PeaceDigits = Peace, WarDigits = War, EmptyDigits = Empty };
+        s.SetItem(0x5001u, 0x99u);
+        s.SetShortcutNum(0, peace: true);
+        Assert.Same(Peace, s.ActiveDigitArray());
+    }
+
+    [Fact]
+    public void ActiveDigitArray_occupiedSlot_warStance_returnsWarDigits()
+    {
+        var s = new UiItemSlot { PeaceDigits = Peace, WarDigits = War, EmptyDigits = Empty };
+        s.SetItem(0x5001u, 0x99u);
+        s.SetShortcutNum(0, peace: false);
+        Assert.Same(War, s.ActiveDigitArray());
+    }
+
+    [Fact]
+    public void ActiveDigitArray_emptySlot_nullEmptyDigits_returnsNull()
+    {
+        var s = new UiItemSlot { PeaceDigits = Peace, WarDigits = War, EmptyDigits = null };
+        s.SetShortcutNum(0, peace: true);
+        Assert.Null(s.ActiveDigitArray());
+    }
 }