fix(unreal): U1 -- Unreal compiles again on L61; CoreGeometry asset section; versioned layout-unit upgrade

Windows MSVC catch-up after abyss L15-L61 (no Unreal there): 64-bit clipmap
shift (C4334), qualified VoxelForgeCore::WorldAssemblySubvoxelsPerVoxel, UHT
metadata and duplicate-signature fixes. Optional CoreGeometry section on
UVoxelStrateDefinition (native caves, lattice, profiled heightfields, named
stamps, rooms/tunnels; off/empty by default), resolved after passage planning.
VoxelForgeAssetVersion 2 converts strate height/transition/gap to metres on
load (exact, 0 m change on the owner's 5 assets; packages never saved).
Gates pass (SurfaceWorld, FloatingIslands, Core-off, M2, W1/W8/W16).
KNOWN REGRESSION vs L14 (2026-10-20, same command, same PC): full set 3.9 ->
5.37 s, worker-seconds 15.8 -> 30, LOD0 1.11 -> 1.69 s, ring 0.50 -> 0.80 s;
not yet attributed (Core L15-L61 vs Unreal side).
Report: Standalone/Research/WorldAssemblerM3/Unreal1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Source/VoxelForge/Public/VoxelSettings.h

@@ -21,6 +21,8 @@ class UVoxelSettings : public UPrimaryDataAsset
 	GENERATED_BODY()
 
 public:
+	virtual void Serialize(FArchive& Ar) override;
+	virtual void PostLoad() override;
 
     /** Season seed/radius replace the mutable authored values only when Season is assigned. */
     int32 GetEffectiveWorldSeed() const;
@@ -411,14 +413,16 @@ public:
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Strates", meta = (ClampMin = "1"))
 	int32 TotalStrates = 10;
 
-	// Vertical SEPARATION between consecutive strates, in chunks of solid bedrock.
-	// 0 = strates touch (just the seals between them). Higher = a thick rock layer
-	// between each biome that the player must dig through at (0,0), and that the
-	// auto-carved passages tunnel across.
-	//   0 → adjacent layers (default) · 1-2 → a real bedrock band · 4+ → deep separation
-	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Strates", meta = (ClampMin = "0", ClampMax = "16"))
+	// Legacy count retained so pre-metre assets still deserialize during the upgrade.
+	// Ancien compte conserve pour lire les assets precedant les metres.
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Strates", meta = (DeprecatedProperty, DeprecationMessage = "Use InterStrateGapMeters."))
 	int32 InterStrateGapChunks = 0;
 
+	// Physical separation between strata; the layout remains aligned to whole layout chunks.
+	// Separation physique; la disposition reste alignee sur des chunks entiers.
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Strates", meta = (Units = "m", ClampMin = "0", ClampMax = "1024"))
+	double InterStrateGapMeters = 0.0;
+
 	// Physical width of one vertical strate-layout chunk. This is independent of the
 	// Core tile width (32^3 voxels) and of the source generator's voxel resolution.
 	// 8 m preserves the owner's current SurfaceWorld extent at 25 cm Core voxels.