feat(a1): .NET 8 solution scaffold + Model + OpcodeTable loader

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

engine/Age.Engine/Sys4/OpcodeTableJson.cs

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+using System.Text.Json;
+using Age.Engine.Model;
+namespace Age.Engine.Sys4;
+public static class OpcodeTableJson
+{
+    public static OpcodeTable Load(string path)
+    {
+        using var doc = JsonDocument.Parse(File.ReadAllText(path));
+        var dict = new Dictionary<int, (string, int)>();
+        foreach (var e in doc.RootElement.GetProperty("opcodes").EnumerateArray())
+        {
+            int op = Convert.ToInt32(e.GetProperty("op").GetString(), 16);
+            string label = e.GetProperty("label").GetString() ?? "";
+            int argc = e.GetProperty("argc").GetInt32();
+            dict[op] = (label, argc);
+        }
+        return new OpcodeTable(dict);
+    }
+}

engine/Age.Engine/Sys4/Paths.cs

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+namespace Age.Engine.Sys4;
+public static class Paths
+{
+    public static string Repo { get; } = FindRepo();
+    public static string Workspace => Directory.GetParent(Repo)!.FullName;
+    public static string Extracted => Path.Combine(Workspace, "extracted");
+    public static string Data1 => Path.Combine(Extracted, "DATA1");
+    public static string GameDir => Path.Combine(Workspace, "姫狩りダンジョンマイスター");
+    public static string Build => Path.Combine(Repo, "build");
+    public static string OpcodesJson => Path.Combine(Build, "opcodes.json");
+
+    private static string FindRepo()
+    {
+        var d = new DirectoryInfo(AppContext.BaseDirectory);
+        while (d != null && d.Name != "age-reimpl") d = d.Parent;
+        if (d == null) throw new DirectoryNotFoundException(
+            "age-reimpl root not found above " + AppContext.BaseDirectory);
+        return d.FullName;
+    }
+
+    /// name(UPPER).BIN -> path; game-dir loose overrides shadow extracted/DATA1 (mirrors paths.scripts()).
+    public static Dictionary<string, string> Scripts()
+    {
+        var d = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        if (Directory.Exists(Data1))
+            foreach (var p in Directory.EnumerateFiles(Data1, "*.BIN"))
+                d[Path.GetFileName(p).ToUpperInvariant()] = p;
+        if (Directory.Exists(GameDir))
+            foreach (var p in Directory.EnumerateFiles(GameDir, "*.BIN"))
+                d[Path.GetFileName(p).ToUpperInvariant()] = p;
+        return d;
+    }
+}