wip(assembler): M3 round 4 -- plans built inside Unreal from the owner's strates; no import needed

SurfaceWorld and FloatingIslands plans generate at startup from the resolved seed,
strate types and heights (hashed, cached). Imported JSON is an opt-in override (off by
default); an invalid override shows an on-screen notice and falls back to legacy.
BuildHost on staged owner assets: SurfaceWorld cross-system checks, islands at owner
height (route, collision, anchors, BP edits), seed 137501, W1/W8/W16, Core-off legacy.
XY-column cache verified (hashes kept); island LOD0 p95 91 ms vs 133 ms sprint deadline.
Not exercised: season-seed path (no Season in staged settings), rendered notice pixels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Source/VoxelForge/Private/VoxelContentManager.cpp

@@ -283,8 +283,14 @@ void UVoxelContentManager::Initialize(AActor* InOwner, UVoxelStrateManager* InSt
     if (Settings)
     {
         FString ManifestJson, ManifestHash, ManifestError;
-        if (Settings->GetWorldAssemblyManifestJson(ManifestJson, ManifestHash, ManifestError)
-            && !ManifestJson.IsEmpty())
+        const AVoxelWorld* VoxelWorld = Cast<AVoxelWorld>(Owner.Get());
+        const bool bHasRuntimeAssembly = VoxelWorld != nullptr
+            && VoxelWorld->GetVoxelForgeCoreWorldAssembly(ManifestJson, ManifestHash);
+        if (!bHasRuntimeAssembly)
+        {
+            Settings->GetWorldAssemblyManifestJson(ManifestJson, ManifestHash, ManifestError);
+        }
+        if (!ManifestJson.IsEmpty())
         {
             FTCHARToUTF8 Utf8Json(*ManifestJson);
             std::string CoreError;