feat(headless): complete deterministic bot command parity

src/AcDream.Core/Spells/RetailSpellTargetPolicy.cs

@@ -0,0 +1,50 @@
+using AcDream.Core.Items;
+
+namespace AcDream.Core.Spells;
+
+public readonly record struct SpellTargetPolicyResult(
+    bool Allowed,
+    string? Message)
+{
+    public static SpellTargetPolicyResult Accept { get; } =
+        new(true, null);
+}
+
+/// <summary>
+/// Pure projection of
+/// <c>ClientMagicSystem::ObjectCompatibleWithSpellTargetType</c>
+/// (0x00567230). The caller owns target lookup and message presentation.
+/// </summary>
+public static class RetailSpellTargetPolicy
+{
+    private const uint SpecialTargetMask = 0x00008107u;
+
+    public static SpellTargetPolicyResult Evaluate(
+        uint localPlayerId,
+        ClientObject target,
+        SpellMetadata spell)
+    {
+        uint mask = spell.TargetMask;
+        uint special = mask & SpecialTargetMask;
+        if (target.ObjectId == localPlayerId && special == 0u)
+            return new(false, "You cannot cast this spell upon yourself.");
+        if (target.StackSize > 1)
+            return new(false, "Cannot cast spell on a stack of items.");
+
+        if (((uint)target.Type & mask) == 0u && special == 0u)
+            return new(false, $"This spell cannot be cast on {target.Name}.");
+
+        // Retail joins the ordinary type match and the 0x8107 special-mask
+        // bypass before these two gates. IsPlayer is PWD bit 3; otherwise the
+        // object must carry BF_ATTACKABLE. Pets are never legal spell targets.
+        var flags = (PublicWeenieFlags)
+            target.PublicWeenieBitfield.GetValueOrDefault();
+        bool playerOrAttackable = (flags
+            & (PublicWeenieFlags.Player
+                | PublicWeenieFlags.Attackable)) != 0;
+        if (!playerOrAttackable || target.PetOwnerId != 0u)
+            return new(false, $"This spell cannot be cast on {target.Name}.");
+
+        return SpellTargetPolicyResult.Accept;
+    }
+}