feat(launcher): stabilize prepared content updates

src/AcDream.Headless/Configuration/HeadlessConfiguration.cs

@@ -27,6 +27,12 @@ internal sealed class HeadlessContentDescriptor
 
     [JsonRequired]
     public string PreparedAssetPath { get; init; } = string.Empty;
+
+    public string? PreparedAssetOverlayPath { get; init; }
+
+    public uint? PreparedAssetBaseRecipeVersion { get; init; }
+
+    public uint? PreparedAssetEffectiveRecipeVersion { get; init; }
 }
 
 // MF-1 (Campaign OP OP7 review fix, 2026-08-11): record, not class — the

src/AcDream.Headless/Configuration/HeadlessConfigurationLoader.cs

@@ -173,6 +173,18 @@ internal static class HeadlessConfigurationLoader
             throw new HeadlessConfigurationException(
                 "process.content requires non-empty datDirectory and preparedAssetPath.");
         }
+
+        bool hasOverlay = !string.IsNullOrWhiteSpace(
+            content.PreparedAssetOverlayPath);
+        bool hasBaseRecipe = content.PreparedAssetBaseRecipeVersion is > 0;
+        bool hasEffectiveRecipe =
+            content.PreparedAssetEffectiveRecipeVersion is > 0;
+        if (hasOverlay != hasBaseRecipe || hasOverlay != hasEffectiveRecipe)
+        {
+            throw new HeadlessConfigurationException(
+                "process.content overlay path, base recipe, and effective recipe "
+                + "must be supplied together.");
+        }
     }
 
     private static void ValidateSession(

src/AcDream.Headless/Hosting/HeadlessProcessContentOwner.cs

@@ -43,15 +43,56 @@ internal sealed class ProductionHeadlessProcessContentFactory
         string datDirectory = Path.GetFullPath(descriptor.DatDirectory);
         string preparedAssetPath =
             Path.GetFullPath(descriptor.PreparedAssetPath);
+        string? overlayPath = string.IsNullOrWhiteSpace(
+            descriptor.PreparedAssetOverlayPath)
+            ? null
+            : Path.GetFullPath(descriptor.PreparedAssetOverlayPath);
         IDatReaderWriter? dats = null;
         IPreparedAssetSource? prepared = null;
         try
         {
             dats = RuntimeDatCollectionFactory.OpenReadOnly(datDirectory);
-            prepared = new PakPreparedAssetSource(
-                preparedAssetPath,
-                dats,
-                diagnostic);
+            if (overlayPath is null)
+            {
+                prepared = new PakPreparedAssetSource(
+                    preparedAssetPath,
+                    dats,
+                    diagnostic);
+            }
+            else
+            {
+                if (descriptor.PreparedAssetBaseRecipeVersion is not > 0
+                    || descriptor.PreparedAssetEffectiveRecipeVersion
+                        != AcDream.Content.Pak.PakFormat.CurrentBakeToolVersion)
+                {
+                    throw new InvalidDataException(
+                        "Layered prepared content does not match the client's recipe.");
+                }
+
+                var baseSource = new PakPreparedAssetSource(
+                    preparedAssetPath,
+                    PreparedAssetCatalogIdentity.From(
+                        dats,
+                        descriptor.PreparedAssetBaseRecipeVersion.Value),
+                    diagnostic);
+                try
+                {
+                    var overlaySource = new PakPreparedAssetSource(
+                        overlayPath,
+                        PreparedAssetCatalogIdentity.From(
+                            dats,
+                            descriptor.PreparedAssetEffectiveRecipeVersion.Value),
+                        diagnostic);
+                    prepared = new LayeredPreparedAssetSource(
+                        baseSource,
+                        overlaySource);
+                }
+                catch
+                {
+                    baseSource.Dispose();
+                    throw;
+                }
+            }
             MagicCatalog magic = MagicCatalog.Load(dats);
             Region region = dats.Get<Region>(0x13000000u)
                 ?? throw new InvalidOperationException(