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/VoxelSettings.cpp

@@ -95,6 +95,7 @@ bool UVoxelSettings::SetWorldAssemblyManifestJson(const FString& InJson, FString
     WorldAssemblyManifestJson = UTF8_TO_TCHAR(CanonicalJson.c_str());
     WorldAssemblyManifestHash = FString::Printf(TEXT("%016llx"),
         static_cast<unsigned long long>(Manifest->contentHash));
+    bUseWorldAssemblyManifestOverride = true;
     MarkPackageDirty();
     OutReport = FString::Printf(
         TEXT("imported native world assembly %s (%d SurfaceWorld plans, %d floating-island plans)"),
@@ -109,8 +110,17 @@ bool UVoxelSettings::GetWorldAssemblyManifestJson(
     OutCanonicalJson.Reset();
     OutContentHash.Reset();
     OutError.Reset();
+    if (!bUseWorldAssemblyManifestOverride)
+    {
+        return true;
+    }
     if (WorldAssemblyManifestJson.IsEmpty())
     {
+        if (bUseWorldAssemblyManifestOverride)
+        {
+            OutError = TEXT("world assembly override is enabled but its manifest JSON is empty");
+            return false;
+        }
         if (!WorldAssemblyManifestHash.IsEmpty())
         {
             OutError = TEXT("world assembly hash is set but its manifest JSON is empty");