Add document tree infrastructure that creates Silo metadata nodes when
a .kc file is opened. Nodes appear under a "Silo" group and represent
the silo/ ZIP directory entries (manifest, metadata, history, etc.).
New files:
- silo_objects.py: SiloViewerObject proxy with Transient properties
- silo_viewproviders.py: SiloViewerViewProvider with icon stubs
- silo_tree.py: SiloTreeBuilder with conditional node creation
- silo_document.py: SiloDocumentObserver singleton + registration
Modified files:
- kc_format.py: pre_reinject hook system for silo/ entry mutation
- InitGui.py: 600ms timer registration for document observer
- CMakeLists.txt: install list for 4 new Python files
Closes#37
