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/Gui/Document.cpp

@@ -1616,7 +1616,7 @@ bool Document::saveAs()
         getMainWindow(),
         QObject::tr("Save %1 Document").arg(exe),
         name,
-        QStringLiteral("%1 %2 (*.FCStd)").arg(exe, QObject::tr("Document"))
+        QStringLiteral("%1 %2 (*.FCStd *.kc)").arg(exe, QObject::tr("Document"))
     );
 
     if (!fn.isEmpty()) {
@@ -1739,7 +1739,7 @@ bool Document::saveCopy()
         getMainWindow(),
         QObject::tr("Save %1 Document").arg(exe),
         QString::fromUtf8(getDocument()->FileName.getValue()),
-        QObject::tr("%1 document (*.FCStd)").arg(exe)
+        QObject::tr("%1 document (*.FCStd *.kc)").arg(exe)
     );
     if (!fn.isEmpty()) {
         const char* DocName = App::GetApplication().getDocumentName(getDocument());