feat(a1): SYS4 XOR-FF cp932 string codec

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

engine/Age.Engine.Tests/Sys4StringCodecTests.cs

@@ -0,0 +1,24 @@
+using Age.Engine.Sys4;
+using Xunit;
+
+public class Sys4StringCodecTests
+{
+    [Fact]
+    public void DecodesXorFfCp932Ascii()
+    {
+        // "AB\0\0" little-endian = 0x00004241, stored XOR 0xFFFFFFFF
+        uint[] dw = { 0x00004241u ^ 0xFFFFFFFFu };
+        var (text, nd) = Sys4StringCodec.Decode(dw, 0);
+        Assert.Equal("AB", text);
+        Assert.Equal(1, nd);
+    }
+
+    [Fact]
+    public void RejectsNonString()
+    {
+        // 0x00000000 XOR-decodes to bytes 0xFF,0xFF,0xFF,0xFF -> no NUL, invalid
+        uint[] dw = { 0x00000000u };
+        var (text, _) = Sys4StringCodec.Decode(dw, 0);
+        Assert.Null(text);
+    }
+}