…cing

Refactors the export logic to map `THREE.Group` objects to 3MF component assemblies.

- Previously, exports were often treated as a single object or split strictly by mesh, making it difficult to manipulate complex multi-mesh items as single units in a slicer.
- Each top-level group is now exported as a distinct build item, and its children are grouped logically using 3MF components.
- This allows slicers to recognize complex assemblies as single manipulatable objects on the plate while preserving their distinct geometry.
- This is a non-breaking change that significantly improves compatibility with slicer workflows.
- Includes general code quality improvements and internal refactoring.

- Replaces coordinate baking with native 3MF component transforms, preserving original geometry and improving modularity.
- Implements recursive transform accumulation to correctly position sub-components in deep hierarchies.
- Updates export logic to treat direct children of THREE.Scene as separate top-level objects while preserving THREE.Group objects as unified assemblies.
- Fixes layout issues where nested groups were previously stacked at the origin.
- Adds initial test suite for mesh, group, and scene export combinations.