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

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

engine/Age.Engine/Age.Engine.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="System.Text.Encoding.CodePages" Version="10.0.9" />
+  </ItemGroup>
+
+</Project>

engine/Age.Engine/Model/Instruction.cs

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+namespace Age.Engine.Model;
+public sealed record Instruction(int Offset, int Opcode, IReadOnlyList<Operand> Args);

engine/Age.Engine/Model/OpcodeTable.cs

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+namespace Age.Engine.Model;
+public sealed class OpcodeTable
+{
+    private readonly IReadOnlyDictionary<int, (string Label, int Argc)> _t;
+    public OpcodeTable(IReadOnlyDictionary<int, (string, int)> t) => _t = t;
+    public int Count => _t.Count;
+    public bool TryGet(int op, out string label, out int argc)
+    {
+        if (_t.TryGetValue(op, out var e)) { label = e.Label; argc = e.Argc; return true; }
+        label = ""; argc = -1; return false;
+    }
+    public string Label(int op) => _t.TryGetValue(op, out var e) ? e.Label : "";
+    public int Argc(int op) => _t.TryGetValue(op, out var e) ? e.Argc : -1;
+}

engine/Age.Engine/Model/Operand.cs

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+namespace Age.Engine.Model;
+public readonly record struct Operand(int Type, long Value);

engine/Age.Engine/Model/Script.cs

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+namespace Age.Engine.Model;
+public sealed class Script
+{
+    public required ScriptHeader Header { get; init; }
+    public required IReadOnlyList<Instruction> Instructions { get; init; }
+    public required IReadOnlyDictionary<int, int> IndexByOffset { get; init; }
+    public required IReadOnlyDictionary<int, string> Strings { get; init; }
+    public string GetString(int offset) => Strings.TryGetValue(offset, out var s) ? s : "";
+}

engine/Age.Engine/Model/ScriptHeader.cs

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+namespace Age.Engine.Model;
+public sealed record ScriptHeader(
+    int LocalInt1, int LocalFloats, int LocalStrings1,
+    int LocalInt2, int Unknown, int LocalStrings2);

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;
+    }
+}