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>

src/AcDream.Core/Items/ToolbarAmmoPolicy.cs

@@ -0,0 +1,47 @@
+namespace AcDream.Core.Items;
+
+/// <summary>
+/// Pure retail toolbar ammunition selection.
+/// Retail references: <c>gmToolbarUI::UpdateAmmoID @ 0x004BF210</c> and
+/// <c>gmToolbarUI::UpdateAmmoNumber @ 0x004BE9E0</c>.
+/// </summary>
+public static class ToolbarAmmoPolicy
+{
+    public readonly record struct Result(uint ObjectId, int DisplayCount)
+    {
+        public bool IsVisible => ObjectId != 0;
+    }
+
+    /// <summary>
+    /// Resolve the first equipped missile weapon. If it is stackable, retail
+    /// treats that stack as its own ammunition (thrown weapons). Otherwise it
+    /// resolves the first separately equipped missile-ammo object.
+    /// </summary>
+    public static Result Resolve(IReadOnlyList<ClientObject> inventoryPlacements)
+    {
+        ArgumentNullException.ThrowIfNull(inventoryPlacements);
+
+        ClientObject? missileWeapon = FindAtLocation(
+            inventoryPlacements,
+            EquipMask.MissileWeapon);
+        ClientObject? ammo = missileWeapon is { StackSizeMax: > 1 }
+            ? missileWeapon
+            : FindAtLocation(inventoryPlacements, EquipMask.MissileAmmo);
+
+        return ammo is null
+            ? default
+            : new Result(ammo.ObjectId, ammo.StackSize == 0 ? 1 : ammo.StackSize);
+    }
+
+    private static ClientObject? FindAtLocation(
+        IReadOnlyList<ClientObject> inventoryPlacements,
+        EquipMask location)
+    {
+        // ACCWeenieObject::GetObjectAtLocation @ 0x0058CE00 returns the first
+        // inventory placement whose location mask intersects the requested mask.
+        foreach (ClientObject item in inventoryPlacements)
+            if ((item.CurrentlyEquippedLocation & location) != 0)
+                return item;
+        return null;
+    }
+}