feat(sdk): add panel provider and theme engine to kcsdk (#352, #353)
- IPanelProvider: abstract interface for dock panels with PySide widget bridging
- PyIPanelProvider/PyProviderHolder: pybind11 trampoline + GIL-safe holder
- WidgetBridge: PySide QWidget → C++ QWidget* conversion via Shiboken
- SDKRegistry: panel registration, creation, and lifecycle management
- ThemeEngine: C++ singleton with minimal YAML parser, palette cache,
getColor/allTokens/formatQss matching Python Palette API
- kcsdk bindings: DockArea, PanelPersistence enums, panel functions,
theme_color, theme_tokens, format_qss, load_palette
- dock.py: kcsdk delegation with FreeCADGui fallback
- theme.py: kcsdk delegation with Python YAML fallback
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