feat(phys L.2g slice 1): inbound SetState (0xF74B) parser

DTO + TryParse for the GameMessageSetState wire message. The server
broadcasts this when an already-spawned entity's PhysicsState changes
post-CreateObject — chiefly when a door's Ethereal bit toggles on Use.

Wire format per holtburger SetStateData (validated against retail-format
servers): u32 opcode + u32 guid + u32 state + u16 instanceSequence + u16
stateSequence = 16 bytes total. Mirrors the existing VectorUpdate.cs
template.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,43 @@
+using System;
+using System.Buffers.Binary;
+using AcDream.Core.Net.Messages;
+using Xunit;
+
+namespace AcDream.Core.Net.Tests.Messages;
+
+public class SetStateTests
+{
+    [Fact]
+    public void TryParse_WellFormedBody_ReturnsParsed()
+    {
+        var buf = new byte[16];
+        BinaryPrimitives.WriteUInt32LittleEndian(buf.AsSpan(0, 4), 0xF74Bu);
+        BinaryPrimitives.WriteUInt32LittleEndian(buf.AsSpan(4, 4), 0x000F4244u); // door guid
+        BinaryPrimitives.WriteUInt32LittleEndian(buf.AsSpan(8, 4), 0x00000004u); // ETHEREAL bit
+        BinaryPrimitives.WriteUInt16LittleEndian(buf.AsSpan(12, 2), (ushort)355);
+        BinaryPrimitives.WriteUInt16LittleEndian(buf.AsSpan(14, 2), (ushort)42);
+
+        var parsed = SetState.TryParse(buf);
+
+        Assert.NotNull(parsed);
+        Assert.Equal(0x000F4244u, parsed.Value.Guid);
+        Assert.Equal(0x00000004u, parsed.Value.PhysicsState);
+        Assert.Equal((ushort)355, parsed.Value.InstanceSequence);
+        Assert.Equal((ushort)42, parsed.Value.StateSequence);
+    }
+
+    [Fact]
+    public void TryParse_Truncated_ReturnsNull()
+    {
+        var buf = new byte[10];
+        Assert.Null(SetState.TryParse(buf));
+    }
+
+    [Fact]
+    public void TryParse_WrongOpcode_ReturnsNull()
+    {
+        var buf = new byte[16];
+        BinaryPrimitives.WriteUInt32LittleEndian(buf.AsSpan(0, 4), 0xF74Cu);
+        Assert.Null(SetState.TryParse(buf));
+    }
+}