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);
+    }
+}

engine/Age.Engine/Sys4/Sys4StringCodec.cs

@@ -0,0 +1,45 @@
+using System.Text;
+namespace Age.Engine.Sys4;
+public static class Sys4StringCodec
+{
+    private static readonly Encoding Cp932;
+    static Sys4StringCodec()
+    {
+        Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+        Cp932 = Encoding.GetEncoding(932);
+    }
+
+    public static (string? Text, int NDwords) Decode(IReadOnlyList<uint> dwords, int start, int limit = 4096)
+    {
+        int n = dwords.Count, end = Math.Min(start + limit, n);
+        var raw = new List<byte>();
+        bool sawNul = false;
+        for (int j = start; j < end; j++)
+        {
+            uint x = dwords[j] ^ 0xFFFFFFFFu;
+            raw.Add((byte)(x & 0xFF)); raw.Add((byte)((x >> 8) & 0xFF));
+            raw.Add((byte)((x >> 16) & 0xFF)); raw.Add((byte)((x >> 24) & 0xFF));
+            if (raw[^1] == 0 || raw[^2] == 0 || raw[^3] == 0 || raw[^4] == 0) { sawNul = true; break; }
+        }
+        if (!sawNul) return (null, 0);
+        int nul = raw.IndexOf(0);
+        byte[] s = raw.GetRange(0, nul < 0 ? raw.Count : nul).ToArray();
+        if (s.Length < 1) return ("", 1);
+        int i = 0, chars = 0;
+        while (i < s.Length)
+        {
+            byte b = s[i];
+            if (b >= 0x20 && b <= 0x7E) { i++; chars++; }
+            else if ((b >= 0x81 && b <= 0x9F) || (b >= 0xE0 && b <= 0xEA))
+            {
+                if (i + 1 < s.Length && s[i + 1] >= 0x40 && s[i + 1] <= 0xFC && s[i + 1] != 0x7F) { i += 2; chars++; }
+                else return (null, 0);
+            }
+            else return (null, 0);
+        }
+        if (chars < 1) return (null, 0);
+        string text;
+        try { text = Cp932.GetString(s); } catch { return (null, 0); }
+        return (text, (s.Length / 4) + 1);
+    }
+}