feat(net): D.2b-B B-Wire — SetStackSize (0x0197) parser

Top-level GameMessage (UIQueue), not a GameEvent. Server sends after a
stack merge / split to update count + value. Layout: opcode(4) + seq(1)
+ guid(4) + stackSize(4) + value(4) = 17 bytes. 3 tests: happy path,
wrong opcode, truncated. Client consumer: ACCWeenieObject::ServerSaysSetStackSize.

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

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

@@ -0,0 +1,37 @@
+using System.Buffers.Binary;
+using AcDream.Core.Net.Messages;
+using Xunit;
+
+namespace AcDream.Core.Net.Tests.Messages;
+
+public sealed class SetStackSizeTests
+{
+    private static byte[] Build(uint guid, int stackSize, int value, byte seq = 1, uint opcode = 0x0197u)
+    {
+        var b = new byte[17];
+        BinaryPrimitives.WriteUInt32LittleEndian(b.AsSpan(0), opcode);
+        b[4] = seq;
+        BinaryPrimitives.WriteUInt32LittleEndian(b.AsSpan(5), guid);
+        BinaryPrimitives.WriteInt32LittleEndian(b.AsSpan(9), stackSize);
+        BinaryPrimitives.WriteInt32LittleEndian(b.AsSpan(13), value);
+        return b;
+    }
+
+    [Fact]
+    public void TryParse_validStack_returnsGuidStackValue()
+    {
+        var p = SetStackSize.TryParse(Build(0x50000A01u, stackSize: 25, value: 125));
+        Assert.NotNull(p);
+        Assert.Equal(0x50000A01u, p!.Value.Guid);
+        Assert.Equal(25, p.Value.StackSize);
+        Assert.Equal(125, p.Value.Value);
+    }
+
+    [Fact]
+    public void TryParse_wrongOpcode_returnsNull()
+        => Assert.Null(SetStackSize.TryParse(Build(1, 1, 1, opcode: 0x0196u)));
+
+    [Fact]
+    public void TryParse_truncated_returnsNull()
+        => Assert.Null(SetStackSize.TryParse(new byte[16]));
+}