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/Public/VoxelWorld.h

@@ -588,6 +588,7 @@ public:
     bool IsVoxelForgeCoreReady() const;
     FVector GetVoxelForgeCoreDefaultCenterWorld() const;
     bool QueryVoxelForgeCoreDensityAt(FVector WorldLocation, float& OutDensity) const;
+    bool GetVoxelForgeCoreWorldAssembly(FString& OutCanonicalJson, FString& OutContentHash) const;
 
     UFUNCTION(BlueprintPure, Category = "Voxel World|Strate")
     int32 GetStrateAtPosition(FVector WorldPosition) const;
@@ -675,7 +676,8 @@ private:
     void SetDensityVolumeMaterialParamsDisabled();
 
     /** Runtime Core snapshot, world lifetime, and worker-side streaming bridge. */
-    bool InitializeVoxelForgeCoreWorld();
+    bool InitializeVoxelForgeCoreWorld(FString& OutFailureReason);
+    void FallBackToLegacyAfterCoreInitializationFailure(const FString& FailureReason);
     void BuildCoreDesiredTiles(const FIntVector& Center, const FVector& PlayerPosition,
                                APawn* PlayerPawn, const FVector& PlayerHeading,
                                TArray<FVoxelTileKey>& OutLeavers);
@@ -740,6 +742,8 @@ private:
     TWeakObjectPtr<UVolumeTexture> LastBoundVolTex0;              // re-push MIDs if the L0 texture was recreated
 
     TSharedPtr<FVoxelForgeCoreRuntimeState, ESPMode::ThreadSafe> VoxelForgeCoreState;
+    FString VoxelForgeCoreWorldAssemblyJson;
+    FString VoxelForgeCoreWorldAssemblyHash;
     FVector CoreFallbackCenterLocalCm = FVector::ZeroVector;
     bool bCoreRegionCompilePending = false;
     TArray<FVoxelTileKey> PendingCoreRegionTiles;