feat(plugin): expose the slicing print-graph as raw orca.host classes + Twistify sample

Adds PluginHostSlicing, which registers the print-graph data model (Print,
PrintObject, Layer, LayerRegion, Surface, ExPolygon, extrusions, ...) into the
orca.host submodule in the same raw-class style as PluginHostApi's Model/Preset
graph, with shared helpers in PluginBindingUtils. SlicingPipelinePluginCapability
is trimmed to the capability surface (the standalone SlicingNumpy helper is folded
away). Adds the Twistify example plugin next to Inset and broadens the binding,
hook, and plugin-install tests.

src/slic3r/GUI/Tab.cpp

@@ -1790,6 +1790,13 @@ void Tab::on_value_change(const std::string& opt_key, const boost::any& value)
         return;
     }
 
+    // Keep this preset's "plugins" manifest in sync when a plugin picker changes, so the edited preset
+    // always carries resolved "name;uuid;capability" references that full_config() and save_to_json()
+    // then pass downstream as-is -- no separate rebuild anywhere else.
+    if (const ConfigOptionDef* opt_def = m_config->def()->get(opt_key);
+        opt_def && opt_def->gui_type == ConfigOptionDef::GUIType::plugin_picker)
+        m_config->update_plugin_manifest();
+
     if (opt_key == "gcode_flavor" && m_type == Preset::TYPE_PRINTER) {
         if (auto printer_tab = dynamic_cast<TabPrinter*>(this))
             printer_tab->on_gcode_flavor_changed();