feat(sdk): add IMenuProvider interface and register_command wrapper (#355)

IMenuProvider: declarative menu placement with optional context awareness.
C++ interface with pybind11 bindings + GIL-safe holder. SDKMenuManipulator
(shared WorkbenchManipulator) injects menu items on workbench switch,
filtered by editing context when context_ids() is non-empty.

register_command(): thin Python wrapper around FreeCADGui.addCommand()
that standardizes the calling convention within the SDK contract.

Python wrappers (kindred_sdk.register_menu, kindred_sdk.register_command)
use kcsdk-first routing with FreeCADGui fallback.

src/Gui/SDK/bindings/kcsdk_py.cpp

@@ -25,14 +25,17 @@
 #include <pybind11/stl.h>
 #include <pybind11/functional.h>
 
+#include <Gui/SDK/IMenuProvider.h>
 #include <Gui/SDK/IPanelProvider.h>
 #include <Gui/SDK/IToolbarProvider.h>
 #include <Gui/SDK/SDKRegistry.h>
 #include <Gui/SDK/ThemeEngine.h>
 #include <Gui/SDK/Types.h>
 
+#include "PyIMenuProvider.h"
 #include "PyIPanelProvider.h"
 #include "PyIToolbarProvider.h"
+#include "PyMenuHolder.h"
 #include "PyProviderHolder.h"
 #include "PyToolbarHolder.h"
 
@@ -284,6 +287,41 @@ PYBIND11_MODULE(kcsdk, m)
         },
         "Return IDs of all registered toolbar providers.");
 
+    // -- Menu provider API --------------------------------------------------
+
+    py::class_<IMenuProvider, PyIMenuProvider>(m, "IMenuProvider")
+        .def(py::init<>())
+        .def("id", &IMenuProvider::id)
+        .def("menu_path", &IMenuProvider::menu_path)
+        .def("items", &IMenuProvider::items)
+        .def("context_ids", &IMenuProvider::context_ids);
+
+    m.def("register_menu",
+        [](py::object provider) {
+            auto holder = std::make_unique<PyMenuHolder>(std::move(provider));
+            SDKRegistry::instance().registerMenu(std::move(holder));
+        },
+        py::arg("provider"),
+        "Register a menu provider for declarative menu placement.\n\n"
+        "Parameters\n"
+        "----------\n"
+        "provider : IMenuProvider\n"
+        "    Menu provider implementing id(), menu_path(), items().\n"
+        "    Optionally override context_ids() to limit to specific contexts.");
+
+    m.def("unregister_menu",
+        [](const std::string& id) {
+            SDKRegistry::instance().unregisterMenu(id);
+        },
+        py::arg("id"),
+        "Remove a registered menu provider.");
+
+    m.def("registered_menus",
+        []() {
+            return SDKRegistry::instance().registeredMenus();
+        },
+        "Return IDs of all registered menu providers.");
+
     // -- Theme engine API ---------------------------------------------------
 
     m.def("theme_color",