feat(ui): port retail toolbar ammo count

Resolve stacked thrown weapons versus separately equipped missile ammo exactly like gmToolbarUI, normalize a zero stack to one, and refresh the authored missile indicator from equipment and stack events. Keep the decision in pure Core with retained and real-DAT conformance coverage.

Co-Authored-By: Codex <codex@openai.com>

tests/AcDream.Core.Tests/Items/ToolbarAmmoPolicyTests.cs

@@ -0,0 +1,65 @@
+using AcDream.Core.Items;
+
+namespace AcDream.Core.Tests.Items;
+
+public sealed class ToolbarAmmoPolicyTests
+{
+    [Fact]
+    public void StackableMissileWeapon_isItsOwnAmmo()
+    {
+        var thrown = Item(1u, EquipMask.MissileWeapon, stack: 7, maxStack: 25);
+        var separateAmmo = Item(2u, EquipMask.MissileAmmo, stack: 80, maxStack: 100);
+
+        ToolbarAmmoPolicy.Result result = ToolbarAmmoPolicy.Resolve(
+            new[] { thrown, separateAmmo });
+
+        Assert.Equal(new ToolbarAmmoPolicy.Result(1u, 7), result);
+    }
+
+    [Fact]
+    public void NonStackableLauncher_usesSeparateAmmoSlot()
+    {
+        var bow = Item(1u, EquipMask.MissileWeapon, stack: 1, maxStack: 1);
+        var arrows = Item(2u, EquipMask.MissileAmmo, stack: 42, maxStack: 100);
+
+        ToolbarAmmoPolicy.Result result = ToolbarAmmoPolicy.Resolve(
+            new[] { bow, arrows });
+
+        Assert.Equal(new ToolbarAmmoPolicy.Result(2u, 42), result);
+    }
+
+    [Fact]
+    public void MissingAmmo_isHidden_andWireZeroStackDisplaysOne()
+    {
+        Assert.Equal(default, ToolbarAmmoPolicy.Resolve(Array.Empty<ClientObject>()));
+
+        var arrows = Item(3u, EquipMask.MissileAmmo, stack: 0, maxStack: 100);
+        Assert.Equal(
+            new ToolbarAmmoPolicy.Result(3u, 1),
+            ToolbarAmmoPolicy.Resolve(new[] { arrows }));
+    }
+
+    [Fact]
+    public void FirstIntersectingInventoryPlacementWins()
+    {
+        var first = Item(1u, EquipMask.MissileAmmo | EquipMask.Held, stack: 10, maxStack: 100);
+        var second = Item(2u, EquipMask.MissileAmmo, stack: 20, maxStack: 100);
+
+        Assert.Equal(
+            new ToolbarAmmoPolicy.Result(1u, 10),
+            ToolbarAmmoPolicy.Resolve(new[] { first, second }));
+    }
+
+    private static ClientObject Item(
+        uint id,
+        EquipMask location,
+        int stack,
+        int maxStack)
+        => new()
+        {
+            ObjectId = id,
+            CurrentlyEquippedLocation = location,
+            StackSize = stack,
+            StackSizeMax = maxStack,
+        };
+}