feat: .kc file format — Layer 1 (format registration)

Cherry-picked from feat/kc-file-format-layer1 (723a8c98d5).
- Document.cpp: checkFileName() accepts .kc extension
- FreeCADInit.py: register .kc import type
- Dialog filters: Save As, Save Copy, Merge, Project Utility
- kc_format.py: DocumentObserver preserves silo/ entries across saves
- InitGui.py: register kc_format observer on startup

src/App/FreeCADInit.py

@@ -420,7 +420,7 @@ App.__cmake__ = globals().get("cmake", [])
 # store unit test names
 App.__unit_test__ = []
 
-App.addImportType("FreeCAD document (*.FCStd)", "FreeCAD")
+App.addImportType("FreeCAD document (*.FCStd *.kc)", "FreeCAD")
 
 # set to no gui, is overwritten by InitGui
 App.GuiUp = 0