feat: add fill bed with object instances

[BenJule]

Summary

Adds a separate Fill bed with instances action alongside the existing Fill bed with copies action.

The existing action continues to create independent object copies. The new action fills the remaining plate area with linked ModelInstance entries belonging to the selected object.

Changes

• Add separate menu entries for:
  • Fill bed with copies
  • Fill bed with instances
• Add a dedicated persistent FillBedJob for instance creation
• Reuse the selected instance as the transformation template
• Add generated instances to the existing ModelObject
• Update the object tree correctly when transitioning from one instance to multiple instances
• Preserve the existing copy-based behavior unchanged

Implementation notes

Bambu Studio already contained the fill_bed_with_instances() entry point, but it invoked the same copy-based fill-bed job.

This change gives the fill-bed job an explicit copies/instances mode and registers separate persistent jobs for both operations.

Testing

• Targeted compilation of FillBedJob.cpp
• Targeted compilation of GUI_Factories.cpp
• Targeted compilation of Plater.cpp
• git diff --check

Not tested:

• Full application build
• Runtime UI behavior

Related to #496