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>

src/AcDream.Core.Net/Messages/SetStackSize.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Buffers.Binary;
+
+namespace AcDream.Core.Net.Messages;
+
+/// <summary>
+/// Inbound <c>SetStackSize (0x0197)</c> — a top-level GameMessage (UIQueue group),
+/// NOT a GameEvent. The server updates a stack's count + value after a merge / split.
+/// Client consumer: ACCWeenieObject::ServerSaysSetStackSize.
+///
+/// <para>Wire layout (ACE <c>GameMessageSetStackSize.cs</c>, size hint 17):</para>
+/// <code>
+///   u32 opcode = 0x0197
+///   u8  sequence    // ByteSequence (UpdatePropertyInt) — not honored
+///   u32 guid
+///   i32 stackSize
+///   i32 value
+/// </code>
+/// </summary>
+public static class SetStackSize
+{
+    public const uint Opcode = 0x0197u;
+
+    public readonly record struct Parsed(uint Guid, int StackSize, int Value);
+
+    /// <summary>Parse a raw 0x0197 body. Returns null on opcode mismatch / truncation.</summary>
+    public static Parsed? TryParse(ReadOnlySpan<byte> body)
+    {
+        if (body.Length < 17) return null;          // 4 + 1 + 4 + 4 + 4
+        if (BinaryPrimitives.ReadUInt32LittleEndian(body) != Opcode) return null;
+        int pos = 4;
+        pos += 1;                                   // sequence byte (not honored)
+        uint guid = BinaryPrimitives.ReadUInt32LittleEndian(body[pos..]); pos += 4;
+        int stack = BinaryPrimitives.ReadInt32LittleEndian(body[pos..]); pos += 4;
+        int value = BinaryPrimitives.ReadInt32LittleEndian(body[pos..]);
+        return new Parsed(guid, stack, value);
+    }
+}