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

Register .kc as a recognized file type alongside .FCStd:

C++ changes:
- Document.cpp: checkFileName() accepts .kc extension
- Gui/Document.cpp: Save As and Save Copy dialogs include *.kc
- CommandDoc.cpp: Merge document dialog includes *.kc
- DlgProjectUtility.cpp: Project utility dialog includes *.kc

Python changes:
- FreeCADInit.py: register *.kc in import type system
- kc_format.py: DocumentObserver that preserves silo/ ZIP entries
  across saves (caches before save, re-injects after save)
- InitGui.py: register kc_format observer on startup

Silo integration:
- get_cad_file_path() generates .kc paths for new files
- find_file_by_part_number() finds both .kc and .FCStd, preferring .kc
- search_local_files() lists both .kc and .FCStd files

The .kc format is a superset of .FCStd with a silo/ directory
containing Kindred platform metadata. See docs/KC_SPECIFICATION.md.

src/Gui/Dialogs/DlgProjectUtility.cpp

@@ -46,7 +46,7 @@ DlgProjectUtility::DlgProjectUtility(QWidget* parent, Qt::WindowFlags fl)
     ui->setupUi(this);
     connect(ui->extractButton, &QPushButton::clicked, this, &DlgProjectUtility::extractButton);
     connect(ui->createButton, &QPushButton::clicked, this, &DlgProjectUtility::createButton);
-    ui->extractSource->setFilter(QStringLiteral("%1 (*.FCStd)").arg(tr("Project file")));
+    ui->extractSource->setFilter(QStringLiteral("%1 (*.FCStd *.kc)").arg(tr("Project file")));
 }
 
 /**