Add selection-scoped CSG difference

[drdator]

Summary

• add Difference for subtracting later-selected brushes from the first selection
• preserve unrelated brushes and support multiple cutters, full removal, undo, and redo
• clarify and regroup the CSG menu with concise names and brush-aware enabled states
• reuse the multi-carver difference helper in the existing Carve operation

Verification

• npm run check
• interactive CSG menu and Difference workflow verification
• focused CSG Difference tests