Add IScriptProvider + Sys4ScriptProvider (call-script id -> Script)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine.Tests/Sys4ScriptProviderTests.cs

@@ -0,0 +1,20 @@
+using Age.Engine.Sys4;
+using Xunit;
+
+public class Sys4ScriptProviderTests
+{
+    [Fact]
+    public void ResolvesKnownIdsToTheirScripts()
+    {
+        var table = OpcodeTableJson.Load(Paths.OpcodesJson);
+        var provider = Sys4ScriptProvider.Load(table);
+
+        var additem = provider.GetById(0x1ab);   // ADDITEM.BIN
+        var mes = provider.GetById(0x2ae7);      // MES.BIN
+        Assert.NotNull(additem);
+        Assert.NotNull(mes);
+        Assert.True(additem!.Instructions.Count > 0);
+        Assert.Same(additem, provider.GetById(0x1ab));   // cached: same instance
+        Assert.Null(provider.GetById(long.MaxValue));    // unknown id
+    }
+}

engine/Age.Engine/Hosting/IScriptProvider.cs

@@ -0,0 +1,10 @@
+using Age.Engine.Model;
+namespace Age.Engine.Hosting;
+
+/// <summary>Resolves a call-script id (a raw SYS4INI file index) to a loaded <see cref="Script"/>.
+/// Implemented in Sys4; injected into the VM so the Vm layer never references Sys4.</summary>
+public interface IScriptProvider
+{
+    /// <summary>The script for this id, or null if the id maps to no known script.</summary>
+    Script? GetById(long id);
+}

engine/Age.Engine/Sys4/Paths.cs

@@ -10,6 +10,7 @@ public static class Paths
     public static string OpcodesJson => Path.Combine(Build, "opcodes.json");
     public static string AssetSectionsJson => Path.Combine(Build, "asset-sections.json");
     public static string AssetIndexJson => Path.Combine(Build, "asset-index.json");
+    public static string CallscriptNamesJson => Path.Combine(Build, "callscript-names.json");
     public static string Textures => Path.Combine(Build, "textures");
 
     private static string FindRepo()

engine/Age.Engine/Sys4/Sys4ScriptProvider.cs

@@ -0,0 +1,39 @@
+using System.Text.Json;
+using Age.Engine.Hosting;
+using Age.Engine.Model;
+namespace Age.Engine.Sys4;
+
+/// <summary>Resolves call-script ids (raw SYS4INI file indices) to loaded scripts, using
+/// build/callscript-names.json (id→name) + Paths.Scripts() (name→path). Cached per id.
+/// The native resolver prefers a loose override before the archive; Paths.Scripts() already
+/// shadows extracted/DATA1 with root overrides, so that behavior is preserved.</summary>
+public sealed class Sys4ScriptProvider : IScriptProvider
+{
+    private readonly OpcodeTable _table;
+    private readonly IReadOnlyDictionary<long, string> _idToName;
+    private readonly Dictionary<string, string> _byName;      // NAME(UPPER) -> path
+    private readonly Dictionary<long, Script?> _cache = new();
+
+    public Sys4ScriptProvider(OpcodeTable table, IReadOnlyDictionary<long, string> idToName,
+                              Dictionary<string, string> byName)
+    { _table = table; _idToName = idToName; _byName = byName; }
+
+    public static Sys4ScriptProvider Load(OpcodeTable table)
+    {
+        var raw = JsonSerializer.Deserialize<Dictionary<string, string>>(
+            File.ReadAllText(Paths.CallscriptNamesJson)) ?? new();
+        var idToName = raw.ToDictionary(kv => long.Parse(kv.Key), kv => kv.Value);
+        return new Sys4ScriptProvider(table, idToName, Paths.Scripts());
+    }
+
+    public Script? GetById(long id)
+    {
+        if (_cache.TryGetValue(id, out var cached)) return cached;
+        Script? s = null;
+        if (_idToName.TryGetValue(id, out var name) &&
+            _byName.TryGetValue(name.ToUpperInvariant(), out var path))
+            s = Sys4Loader.Load(path, _table);
+        _cache[id] = s;
+        return s;
+    }
+}