Hold object lock per placement

- Keep the object read lock across the bake lookup, block pass, grounding and stilt pass so a placement reads one consistent snapshot

core/src/main/java/art/arcane/iris/structure/object/IrisObjectPlacementRunner.java

@@ -475,6 +475,8 @@ final class IrisObjectPlacementRunner {
         int topLayer = Integer.MIN_VALUE;
         int vacuumLowest = Integer.MAX_VALUE;
         int vacuumHighest = Integer.MIN_VALUE;
+        self.readLock.lock();
+        try {
             ObjectPlacementBake bake = ObjectPlacementBake.of(self, spin, ceilingHang, translating ? translateOffset : null);
             PlacementStates palette = new PlacementStates(bake.palette(), config.getRotation(), spinx, spiny, spinz,
                     preventingDecay, placer.isDebugSmartBore(), excludedStiltMaterials);
@@ -819,6 +821,9 @@ final class IrisObjectPlacementRunner {
                     }
                 }
             }
+        } finally {
+            self.readLock.unlock();
+        }
 
         if (vacuuming && vacuumLowest != Integer.MAX_VALUE && placer.getEngine() != null) {
             vacuumTerrain(placer, config, x, z, placementBounds.minX(), placementBounds.maxX(),