test(B.5): exercise i32 sign-correctness for BuildPickUp.placement

The original test only used placement=0, which encodes identically
under WriteInt32 and WriteUInt32. Add a -1 case so a future regression
to the unsigned writer would actually fail the test.

Flagged by Task 1 code review.

tests/AcDream.Core.Net.Tests/Messages/InteractRequestsTests.cs

@@ -66,4 +66,22 @@ public sealed class InteractRequestsTests
         Assert.Equal(0,
             BinaryPrimitives.ReadInt32LittleEndian(body.AsSpan(20)));
     }
+
+    [Fact]
+    public void BuildPickUp_NegativePlacement_WritesSignedLittleEndian()
+    {
+        // Sign-correctness guard: placement is i32 on the wire (ACE
+        // GameActionPutItemInContainer.Handle reads ReadInt32). A
+        // placement=0 test would pass even if the builder used
+        // WriteUInt32, so we also exercise a negative value where the
+        // unsigned/signed encodings would diverge.
+        byte[] body = InteractRequests.BuildPickUp(
+            gameActionSequence: 1,
+            itemGuid:           0x1u,
+            containerGuid:      0x2u,
+            placement:          -1);
+
+        Assert.Equal(-1,
+            BinaryPrimitives.ReadInt32LittleEndian(body.AsSpan(20)));
+    }
 }