Refactor terrain data handling and add comprehensive unit tests for brush, mesh, and serialization.

src/runtime-three/runtime-scene-build.ts

@@ -891,12 +891,12 @@ function buildRuntimeTerrain(
     sampleCountX: terrain.sampleCountX,
     sampleCountZ: terrain.sampleCountZ,
     cellSize: terrain.cellSize,
-    heights: [...terrain.heights],
+    heights: terrain.heights,
     layers: terrain.layers.map((layer) => ({
       materialId: layer.materialId,
       material: resolveRuntimeMaterial(document, layer.materialId)
     })),
-    paintWeights: [...terrain.paintWeights]
+    paintWeights: terrain.paintWeights
   };
 }
 

src/viewport-three/viewport-host.ts

@@ -9701,7 +9701,7 @@ export class ViewportHost {
 
     const committed =
       this.terrainBrushCommitHandler?.({
-        terrain: cloneTerrain(finalPreviewTerrain),
+        terrain: finalPreviewTerrain,
         commandLabel: getTerrainBrushCommandLabel(toolState.tool),
         tool: toolState.tool
       }) === true;